Book of the Dead is a racial class advancement item in YGGDRASIL.
In YGGDRASIL, it was possible to change races, but in order to achieve a racial advancement, one must need to fulfill several conditions and the this would be irreversible. Thus, acquiring specific items are actually necessary in order to shift from one race to another.
Written in Japanese, there was a chapter from the Book of the Dead concerning the transformation of the dead into souls; specifically, the section about differentiation. The book itself almost seems unreadable by New World inhabitants like Fluder unless they used some sort of item or a translation spell to decipher it.
The Book of the Dead is a fairly ancient volume, which gave off a musty smell. Surprisingly enough, the book itself was very sturdy, with no trace of being worm-eaten.
The Book of the Dead allows the player to advance his or her racial class into an Elder Lich. It also contains lore concerning the transformation of the dead into souls.