This work of ineffable Evil is meat and drink to clerics of that alignment. 

To fully consume the contents requires 1 week of reading, but when such has been accomplished, the Evil cleric will gain 1 point of Wisdom and experience points sufficient to place him or her exactly half way into the next level of experience. 

Clerics neither Good nor Evil who read the book will either lose 30,000-120,000 experience points or become Evil without benefit from the work; there is a 50% chance for either. Good clerics perusing the pages of the unspeakable Book of Vile Darkness will have to save versus poison or die; and if they do not die they must save versus magic or become permanently insane. In the latter event, even if the save is successful, the cleric loses 250,000 experience points, less 10,000 for each of his or her points of Wisdom. Other characters of Good alignment will take 5-30 hit points of damage from handling the tome and if they look inside there is an 80% chance a night hag (q.v.) will thereafter come to the character that night and attack. Non-Evil Neutral characters will take 5-20 hit points of damage from handling the book, and reading its pages will cause them to save versus poison or become Evil, immediately seeking out an Evil cleric to confirm their new alignment. (Cf. Book of Exalted Deeds for other details.)