The Book of Bones is an item that increases attack by 10%. It’s a tome with an eye on it.
This book has one of the eyes of a slain monster on its cover. It’s owner carried it with him everywhere he went, and used the book to record everything that he had seen, experienced and done by showing its eye to whatever caught his fancy. It was buried along with the Staff of Bones, perhaps to see if a hero would use it without becoming corrupted by power.