This quest is given by Neloth, who asks the Dragonborn to collect a certain Black Book. This quest is repeatable for as many Black Books there are.
Have you found any other black books?:"Yes. Hermaeus Mora is devious, but then so am I. Now, it's only a rumor of a whisper of a conjecture. But I think this is the place. Now, hurry. That book might contain the final secrets of Xarxes, for all we know."
Upon locating the book, return to Neloth. He remarks:
I have the black book:"Excellent! I'll just make a copy for myself. It's far too dangerous to carry the real book around. Of course, I'm sure you can handle yourself. Of course you can. Of course. Now, take this for your efforts, and we'll call the matter closed."
As well as the reward from the Black Book, the Dragonborn will get 1000 gold when returning it to Neloth. He will not take the book from the Dragonborn, just make a copy of his own, leaving the Dragonborn free to return to Apocrypha whenever desired, to change the rewards from the book.