The Egyptian Book of the Dead (or the Book of Going Forth by Day, as it is often called today) refers to the texts found on numerous scrolls stretching across a period of over a thousand years, with contents that have connections to the earlier Coffin Texts painted on the sarcophagi and even earlier Pyramid Texts carved into the walls of certain surviving ancient pyramids. 

The texts found on these scrolls are not always the same "chapters" of the Book of the Dead -- there is no single "Book of the Dead," but rather many variants found on many different scrolls, each containing a selection of textual chapters (sometimes called "spells") and pictoral "vignettes."  Some of the chapters are more commonly used than others, and all of them are usually referred to by chapter numbers first established by Karl Richard Lepsius (1810 - 1884) in 1842.  You can find a listing of the chapters here.

One feature found in multiple scrolls are various Hymns to the Sun, which Lepsius generally categorized as Chapter 15 of the Book of the Dead.  There are Hymns to Rising Sun and Hymns to the Setting Sun.  A discussion of the Sun Hymns and an example of a Hymn to Ra in his Rising can be found here.  In Lost Light: An Interpretation of Ancient Scriptures (1940), discussed in this previous post among others, and available here for reading in its entirety online, Alvin Boyd Kuhn focuses on some lines from a version of "a Hymn to the Setting Sun" found in the Book of the Dead papyrus of the Eighteenth Dynasty scribe Nekhtu-Amen.

The entire text of this version of "a Hymn to the Setting Sun," translated into English by E. A. Wallis Budge (1857 - 1934), can be found beginning at the bottom of page 11 and continuing through the first part of page 13 in this 1901 collection of texts from the Book of the Dead, all translated by Budge and prefaced by an introduction from Epiphanius Wilson (1845 - 1916).  It reads, in part:
The Osiris of the gods goeth as a leader through the That (underworld), he crasheth through mountains, he bursteth through rocks, he maketh glad (?) the heart of every Khu.  This composition shall be recited by the deceased when he cometh forth and when he goeth in with the gods, among whom he findeth no opposition; then shall he come forth by day in all the manifold and exceedingly numerous forms which he may be pleased to take. [The Osiris . . . saith:] "A hymn of praise to Ra at eventide [when] he setteth as a living being in Baakha.  The great god who dwelleth in his Disk riseth in his two eyes and all the Khus of the underworld receive him in his horizon of Amentet; they shout praises unto Heru-khuti (Harmachis) in his form of Tem, and they sing hymns of joy to Ra when they have received him at the head of his beautiful path of Amentet."  He (i.e., the deceased) saith: "Praise be unto thee, O Ra, praise be unto thee, O Tem, in thy splendid progress.  Thou hast risen and thou hast put on strength, and thou settest like a living being amid thy glories in the horizon of Amentet, in thy domain which is in Manu.  Thy uraeus-goddess is behind thee; thy uraeus-goddess is behind thee.  Hail to thee, in peace; hail to thee, in peace.  Thou joinest thyself unto the Eye of Horus, and thou hidest thyself within its secret place; it destroyeth for thee all the convulsions of thy face, it maketh thee strong with life, and thou livest.  It bindeth its protecting amulets behind thy members.  Thous sailest forth over heaven, and thou makest the earth to be stablished; thou joinest thyself unto the upper heaven, O Luminary.  The two regions of the East and West make adoration unto thee, bowing low and paying homage unto thee, and they praise thee day by day; the gods of Amentet rejoice in thy splendid beauties.  The hidden places adore thee, the aged ones make offerings unto thee, and they create for thee protecting powers.  The divine beings who dwell in the eastern and western horizons transport thee, and those who are in the Sektet boat convey thee round and about.  The Souls of Amentet cry out unto thee and say unto thee when they meet thy majesty (Life, Health, Strength!), 'All hail, all hail!'  When thou comest forth in peace there arise shouts of delight to thee, O thou lord of heaven, thou Prince of Amentet.  Thy mother Isis embraceth thee, and in thee she recognizeth her son, the lord of fear, the mighty one of terror.  Thou settest as a living being within the dark portal.  Thy father Tatunen lifteth thee up and he stretcheth out his two hands behind thee; thou becomest a divine being in the earth.  Thou wakest as thou settest, and thy habitation is in Manu.  Grant thou that I may be venerated before Osiris, and come thou [to me], O Ra-Tem.  Since thou has been adored [by me] that which I wish thou shalt do for me day by day.  Grant thou victory [unto me] before the great company of the gods, O Ra who art doubly beautiful in thy horizon of Amentet, thou lord of Maat who dwellest in the horizon.  The fear of thee is great, thy forms are majestic, and the love of thee is great among those who dwell in the underworld."
All parentheticals, ellipses, and bracketed interpolations in the above quotation are found in the original passage as published by Budge in 1901.

The modern reader may find the above Hymn to the Setting Sun to be extremely obscure and difficult to interpret, and may wonder what on earth this text found in a tomb and written down over 3,200 years ago (probably from material that is even older) has to do with his or her life today!

If the modern reader has been exposed to conventional education at any level, he or she will probably also have been told numerous times and with a great degree of certainty and finality that the Book of the Dead described the journey of the soul in the afterlife, and contained spells to help the soul navigate the mysterious realms it would encounter after death.  Such opinions will only add to the opinion of most readers that the chapters of the Book of the Dead may have some cultural interest to those studying the strange beliefs of the ancient Egyptians, but nothing of practical value to anyone who does not believe that the afterlife looks anything like the one Egyptians imagined and described in their texts and their art.

Alvin Boyd Kuhn argues, however, that these modern teachings fall into the trap of literalism, and that the ancient Egyptian texts were not primarily concerned with an imagined afterlife at all: they were intended to convey profound truths about this life, and they used exquisite and carefully-conceived metaphorical and allegorical vignettes in order to do so.  In his radical interpretation, the texts in the Book of the Dead, like all other ancient sacred scriptures and all the other sacred traditions preserved around the globe, were designed to restore sleepy or amnesiac human beings to full consciousness, urgently and brilliantly presenting a vision of the human condition and the meaning of this life which the ancients believed we forget to some degree in each of our successive incarnations in the material world.

Thus, to the degree that one is interested in the question of human consciousness and what we are doing here in this life, these texts from so many hundreds of centuries ago are as pertinent today as they were when they were sealed in a tomb and slowly covered over the ages by the blowing sands of Egypt.  

But, to understand what they are trying to tell us, we must understand the language of the metaphor which the ancients were using.  One of the keys to doing so is to understand the central metaphor of descent into this life in the world of matter on the part of our spiritual nature as a kind of a "death."  This argument, which Alvin Boyd Kuhn discusses throughout the six hundred-plus pages of Lost Light and supports with overwhelming amounts of evidence, is discussed in part in the previous post entitled "The horizon and the scales of judgement," and we can immediately see that the important concept of the horizon is found in the above quoted Hymn to the Setting Sun as well.  

One of the metaphors which the ancients used in order to convey this vision of human existence as a cycle of incarnation in matter followed by re-ascent into the spiritual realm was the cycle of the heavenly bodies, including that of the sun, which plunges nightly beneath the horizon (matter) only to rise again each morning clothed in fire into the upper realm of the heavens. 

In the above hymn from the Book of the Dead, Alvin Boyd Kuhn zeroes in on just a few of the lines, found towards the end of the quoted passage, specifically: "Thou settest as a living being within the dark portal; . . . thou becomest a divine being in the earth.  Thou wakest as thou settest . . ." (Kuhn, 512).  These assertions Kuhn describes as "wisdom of transcendent importance."  He goes on to explain why:
The declarations of ancient wisdom that we are divinized on earth and that the soul awakes as it sets, or incarnates, are mighty items of knowledge for benighted morals.  But it has been set forth that the descent is a swoon and a going into oblivion, the very sleep of "death."  Now it is pictured as an awakening.  Here again is exemplified the doubleness of esoteric methodology in picturing the two aspects and movements of being.  But the paradox in all these reversals of imagery is readily solvable.  The soul does fall under a spell of Lethe when enshrouded in dense body; nevertheless it finds in that very state the beginning of its true awakening to a higher sense of reality than ever before.  This world is "the place of establishing forever," of bringing purely latent capacity to dynamic realization.  There is involved here the ultimate mystery of life, which is the necessity of the soul's "death" in matter to gain a new birth.  512-513.
The ultimate mystery of life!  That sounds like something that might be pertinent even to modern readers!  

What Kuhn is describing as this "ultimate mystery" is the question of why a soul would incarnate in this physical material world at all, and the answer expressed in this hymn from the Book of the Dead is that the soul awakens when it "sets" (enters the material world), it becomes a living being in the dark portal of the underworld (this incarnate life), it becomes "a divine being in the earth."  Somehow, the ancients taught that there are lessons and experiences which the soul can only profit from when enfleshed in physical matter, subjected to the pesky laws of physics, and placed on this earth to interact with other beings in the material realm.

Elsewhere in Lost Light, Kuhn elaborates further on this teaching, citing other ancient sources to back up his assertions, including the ancient philosopher Plotinus (AD 205 - AD 270), whose teachings have been discussed in previous posts (well worth revisiting) such as this one, this one, and this one.  Kuhn explains the importance of soul's coming into this material world on page 190 of Lost Light, saying:
The world took form upon the model of divine ideas, Plato affirms.  In us men a god is striving to stamp his lines of beauty and grace upon the features of an animal!  The God-word was fleshed so that it could preserve and finally transfigure the mummy with its splendor.  But -- and let ultra-idealists be advised! -- spirit had to have plastic matter upon which to imprint its form and comeliness, else it would have remained forever unknown.  The visible manifestation of latent wisdom, power and love could be achieved only by the spirit's encasement in a body.  Matter, so derided by extreme "spiritual" theory, is the womb in which alone divine conceptions can be brought to birth.  So that the fleshing of soul works  the miracle of its own anointing.  Flesh is the way and the means by which man, the divine thought, is christened with an ever fuller measure of the oil of beatification.
Note that Kuhn was writing in 1940 and the convention then was to say "man" or "men" to include all men and women, as in "mankind" or "humanity," and he specifically states in other passages that his assertions apply to all men and women, and that in fact bifurcation into the two different sexes is only a temporary condition that is part of this physical incarnation -- the soul in its disincarnate form is androgynous (see for instance the discussion on 229 and on pages 498-512).  He refers to the soul as "she" in the following passage, which also asserts the importance of incarnation for the soul to discover truths which could never be learned without taking on a form of matter:
The earth is thus the place of critical interest in the whole cycle.  The life wave is sent forth to return with a harvest of more abundant life.  Now it is only as spirit contacts and overcomes the inertia of matter that it brings its own potentialities to birth.  Abiding eternally on its own plane, as Platonic philosophy says, it remains non-productive.  It must go forth, seek adventure, meet with opposition, wrestle with the powers that would choke it, and achieve its new cyclical victory in a world of adversity.  As Plotinus writes, "It is not enough for the soul merely to exist; she must show what she is capable of begetting."  Here is the model and the genius of all romance, all drama.  And the earth is the scene of this conflict between the embryonic immortal and the titanic mortal forces.  And where the earth stands in the chain of planetary bodies, the physical body of man stands in the chain of vehicles or vestures which compose each individual.  The human body is the seat and arena of the great conflict of personal destiny.  Without dwelling in and mastering the body of flesh, the individual soul, as says Plotinus, would never know her powers.  She would be spiritual, as she was from the start; but she would dream her existence away without ever becoming consciously aware of her latent creative capabilities, if she did not incarnate.  Incarnation is evolution's method of setting the seal of reality upon conscious life.  This is the office of earth-life in the cycle and of incarnation for the individual soul.  And it is the crucial point in all philosophy, as it is the critical point in individual destiny.  As for the soul her pathway to heaven runs through earth, and on it she goes to her "death" to be born anew.  284.
Thus the Hymn to the Setting Sun, seemingly so obscure and so specialized to the belief-system of such a distant and mysterious civilization as that of ancient Egypt, becomes immediate and of tremendous importance to every individual inhabiting a body on this earth today!  It, along with the other ancient teachings found in every part of the globe, concerns our true nature (which we are constantly in danger of forgetting or never even learning), and our purpose for coming here in the first place.  "There is involved here the ultimate mystery of life."