Apparently, there are seven black books, but I can only find six. Online I find that the last book is given by Neloth and the quest is called The Hidden Twilight. I've talked to Neloth and done quests for him until he gives no more of them. I've also beaten the main storyline. Can anyone tell me how to start this quest?

After one of his quests (i think its the one were you find him a new butler) he unlocks the staff enchanter for you, the book is within that room.