This is a newer translation of a book which has been popularly known as "The Tibetan Book of the Dead" in the West for the better part of a century now. The translator/editor takes great pains to explain the inaccuracy of this title and other parts of older translations, based mostly on the misunderstandings of well-meaning Western scholars. Also included is a summary of Tibetan history and culture as it relates to Buddhism, as well as numerous background notes on Buddhist terms and principles.
In short, the true name of this "teaching" or "protocol" (not really a book in the traditional sense, but it's the best we can do with English) is the Book of Natural Liberation Through Understanding in the Between. Essentially, it is an instruction manual for the living on what to do when someone dies, so that the deceased might pass on either to a good rebirth/reincarnation, or even escape the cycle of death/rebirth entirely. The "Between" is the psychic/spiritual interval that comes after physical death and prior to rebirth or escape.
It is important to note here that this work is reflective of the Tibetan school of Buddhism, which is merely one type of Buddhism among many. Tibetan Buddhism has risen considerably in the non-Buddhist public conscience thanks to several factors, like the Chinese occupation of Tibet and the eloquence of the Dalai Lama, but it by no means encompasses what all Buddhists believe.
A quick synopsis of what happens upon death: When you die, your spirit/essence/soul does not immediately depart this world. It enters the Between, where it will stay for anywhere from a few days to 49 days, depending on the level of enlightenment you had while alive. Thus a lama might stay in the Between for a very brief period of time, but a lay person with no spiritual training might be there much longer. Fortunately, this manual is designed to guide deceased persons of any level through into the next life. Basically, you, the reader, act as a kind of psychopomp.
The text is meant to be read in the presence of the dead person's former body. It is believed that upon death, your spirit continues to hover about, confused and bewildered about what just happened to it. It is aware of its surroundings, but in the same way you might be aware of what is happening in a dream--some things are highly distorted and magnified, while others are remarkably clear and lucid. You might see and hear the lamentations of your loved ones, but not understand that you cannot interact with them, for instance. This can lead to suffering and more confusion for the spirit, which is why mourning is not allowed in the presence of the body.
As the days go by, the text changes and assumes different points on the spirit's journey through the Between. At this point things become highly symbolic and mystical, and there is much debate on what precisely is going on, and what Tibetan practitioners believe it represents. I read that Jung took great interest in this book as a kind of proto-psychology, with the different stages of the journey representing emotional states of mind. Encountering "hellish" deities or realms during the journey represent your own guilt, shame, or unreleased hatreds, for example. Successfully navigating these stages is akin to letting go of such negative concepts. Indeed, there are places in the text where this appears to be acknowledged, since the reader urges the dead to remember that all of these terrifying visions exist only in your mind.
At the same time, this translation cautions the Western reader not to sway too far in either direction when it comes to these interpretations. Trying to fully spiritualize or rationalize the Between is not something the text really allows, and trying to do so is probably more a reflection of yourself and your background than on what any of this "really" means. Thurman makes a helpful point when he states that "in your mind" means something very different to a Tibetan Buddhist than it does to many of us. I am reminded of that line in the Deathly Hallows where Dumbledore is speaking to Harry: "Of course it is happening inside your head, Harry. But why on earth should that mean it is not real?"