Lost Knowledge

Walkthrough
There actually are more black books on Solstheim, and an intrepid explorer will probably already have found and explored them all before defeating Miraak. But just in case these have not been found yet, Neloth provides assistance by pointing out where these books can be found, after The Gardener of Men.

Retrieve the Black Book
The Dragonborn is sent to one or more of these locations to retrieve a black book: White Ridge Barrow, Benkongerike, and/or Raven Rock Mine.

Go there, explore, and then retrieve the book.

Talk to Neloth
Return to Neloth with the book, collect the gold reward.

Note: If the Dragonborn has already found all books, Neloth will comment that all have already been found, and that is it.

Verlorenes Wissen