I finally arrived in hell (deep underground) for the first time. When I arrived there i fought two demons, separated by a thin layer of ash blocks.
Even though it didn't do much damage, I attacked them repeatedly with the viletorn, and finally one died.
As the loot fell, my mouse was on it, and I saw an item that seemed to be called: Book of Demon in orange text.
Was I just imagining things? I tried to jump down, but was quite too high and died on falling without reaching the book. Can someone tell me if a book like that really exists?