The Tibetan Book of the Dead. Ancient Tibetan version translated to English was not complete in most of the authors works who tried to. Finding one of the book's translations complete in your own mother tongue would be nice.
Although, for a safer path to take to get a better understanding of it I direct your attention to a course in the college at one of the most prestigious Universities and the class I would recommend is aimed at becoming a linguistics expert, then study the Tibetan's Languages.
I note that the University is no longer in existence. Good luck finding anyone teaching classes in Linguistics that you can get fired up to talk about The Tibetan Book of the Dead. It's probably a fiction anyway.