"I might think twice before placing something like that on my bookshelf." -Tooken Buckleberry.
The Book of Forbidden Truths is an Artifact created by Elfondo, the Spirit of the Depths, resembling a Book of Shadows, though slightly larger and with slightly moving and glowing text. It contains all knowledge, and reading it can uncover the answer to any question the owner might have. Every time the Book is used, the player using it rolls an Intelligence check, with the difficulty determined by the amount of existing lore on the desired answer. After this, the player rolls a d4. If the die lands on a 1, the reader rerolls the die. If the die lands on a 2, the reader suffers short term madness. If the die lands on a 3, the reader suffers long term madness. If the die lands on a 4, the reader suffers indefinite madness. The nature of the reader's madness is determined by a d100 roll, as described in The Dungeon Master's Guide.