Buddhism, which originated in India, reached a height of consciousness exploration in Tibet. All states of existence for the Tibetan Buddhists other than pure Nirvana are reflections of the limited illusion of self-consciousness.

The Tibetan Book of the Dead is a major document within this tradition; It stands alongside Dante's Divine Comedy as one of the great literary works on the afterlife. But while Dante's work is a fiction designed for moral instruction, the Book of the Dead offers a quite literal description of what happens to the mind at death - a user's guide, as it were, to the post-mortem state.

Interestingly enough, the Tibetan Book of the Dead, has been used as a "constant companion" by Carl Jung whose psychological theories tend to unite scientific logic with mysticism. It was also used by Timothy Leary, Richard Alpert and Ralph Metzner as the basis for understanding the nature of the psychedelic experience.

The full Book of the Dead offers a lyrical insight into the Tibetan Buddhist cosmology and its teachings on the nature of mind and consciousness, death and reincarnation. There are chapters on ritual, acts of confession and "consciousness transference". Rhapsodic paeans to the various deities of Tibetan Buddhism jostle with folklore about the signs of impending death ("If the hair on the nape of the neck grows upwards, this indicates death after three months") and the rituals prescribed for averting it.

The journey through the after-death states is described as a series of vivid encounters and revelations; most are, frankly, terrifying. But at each stage, the book offers the consolation that "liberation" can be attained if one is sufficiently primed to recognize and seize the opportunity. The first, briefest, stage is the moment of death itself, when a clear light of "inner radiance" dawns in the dying mind. If one is able to recognize this light as the true nature of mind - and merge with it - then one is immediately "liberated", which is to say, spared the journey towards rebirth. If not, one is thrown into the next, intermediate state.

Here the deceased is confronted initially with a series of visions of the 100 "wrathful and peaceful deities" of the Tibetan Buddhist pantheon, each exactingly described. The lord Vajra Heruka, for example, is "dark blue in colour, with three faces, six arms and four legs", carrying a skull-cap, an axe and a ploughshare. "The female Vajrakrodhisvari is embracing his body, her right hand clasped around his neck and her left offering a skull-cap filled with blood to his mouth…"

These visions are not held to have an inherent reality; rather they are representations of different aspects of the enlightened mind - images depicted in pictures and wall-hangings in temples, which the dying person will be assumed to have meditated on throughout their lifetime. Recognizing these images, the text says, is "like recognizing your own mother", and presents a further opportunity to merge with the primordial state of being and thus be spared the anguish of rebirth.

However, those clouded by confusion and weighed down by the negative inheritance of their past actions are doomed to roam further downwards into the intermediate state. Now, the subtle mental body of the deceased will experience the sense of being able to move unobstructed through earth, boulders and mountains; they will see their home and their grieving relatives, "as if in a dream", and, realizing that they have died, experience an overwhelming suffering "like a fish writhing on hot sand".

Then comes the "life review" in which the "innate good conscience" of the deceased will gather together all of their virtuous actions, counting them out with white pebbles, and the "innate bad conscience" will count out non-virtuous actions with black pebbles. "At this moment you will tremble with extreme fear, awe and terror… In the mirror of past actions [all your virtues and non-virtues] will be reflected vividly and precisely. Your attempts at deceit will be no use. Tying a rope around your neck, Yama [Lord of Death] will drag you forward. He will sever [your head] at the neck, extract your heart, pull out your entrails, lick your brains, drink your blood, eat your flesh and suck your bones. Despite this, you will not die."

At length, reeling from these torments, the deceased will arrive at the portal of rebirth. The book offers advice on "Obstruction of the Womb Entrances" - a sort of last chance saloon - to prevent this eventuality. If not heeded, then the deceased is drawn to seek rebirth in one of the six "realms" of existence - as a god, a demigod, a human, an animal, an anguished spirit or a hell-being, according to their past actions. Those destined for the human realm will be drawn to a man and woman in the act of sex, experiencing the "co-emergent delight" in the midst of the meeting between sperm and ovum, "until finally you will emerge [from the womb] and open your eyes".

Coleman believes that "The Great Liberation by Hearing" can be read as "a metaphor for our daily experience". But what is also striking, he says, are the correspondences to contemporary accounts of the near-death experience. "The meeting with the light, the ability to hear, to pass through material objects, and the life review, which I think is the most critical part. The poet Heathcote Williams came up with this beautiful phrase, 'death develops life's photographs', and this seems to be the critical lesson of the book, that it is imperative to understand the consequences of one's actions."

It is the custom of classical Buddhist texts to emphasize their own importance in tones of a salesman's stridency - "If you have to read just one teaching, make it this one!" The Tibetan Book of the Dead is no exception. The book stresses the importance, too, of committing the text to memory, suggesting it should be read aloud three times a day and so clearly impressed to mind "that even if one were to be pursued by 100 assassins its text and meaning would not be forgotten".

A luxury that few modern readers could afford, perhaps. But Coleman insists that its value goes far beyond an academic interest in an esoteric subject. "The Dalai Lama said something very interesting; he said, if we're going on holiday somewhere we buy a map and a guidebook, so we know where we're going and we know what to expect. Then he burst out laughing and said, it's funny, isn't it? People do that when they're going on holiday, but they don't want to do it when they know they're going to die. This is that guidebook."