It describes the stages of death from a Tibetan's point of view. These stages include the period at the moment of death and the 49 day interval before rebirth.
The book was composed by a monk in the 8th century and is often recited today by a lama (Buddhist spiritual leader) to a recently deceased person in order to help him understand his experiences and gain enlightenment.
Leonard Cohen, the Canadian singer, songwriter and poet, recites the Tibetan Book of the Dead in this 50 minute video.
I found the video on Open Culture and they summarize the video like this:
The film gives us an intimate look at this ceremony, performed after the death of a villager—with its intricate rituals and ancient, unbound, hand-printed text of the book—and touches on the tricky political issues of Buddhist practice in largely Chinese-controlled Tibet. In this first installment above, The Tibetan Book of the Dead: A Way of Life, the Dalai Lama weighs in with his own views on life and death (at 33:22).