Black tome gives an unbiased random spell - Wish and Darkness are equally likely. Same as ! of Wonder, not same as floor books or \ of Wonder.
Corruption gained is 20 CPs per turn spent reading, only applied if you finish reading (so get interrupted, and ALWAYS read in a half time room). Corruption is at least 200 and capped at 6666.
Black tome cannot be destroyed by reading.
In all other respects black tome functions as any other spellbook. Worse, since it can't be blessed.