What does the black book with a red pentagram do?
I was on a run and entered a devil room, for 2 red hearts was a black book with a pentagram on it. I didn't want to sacrifice the 2 hearts for an item I didn't need or possibly didn't want.
Does anyone know what it does?
Its the satanic bible, on usage, gives you a black heart
If you mean this one:
it's the Satanic Bible, and gives Isaac a black heart when used. A behaviour similar to the Book of revelations.
It's an evil item, so probably it increases the chance of spawning a devil deal room.
Source here.