Welcome! The laws regarding derivative works and fair use are somewhat hazy, and they vary by jurisdiction. That is, what is legal here in the United States might not be in Switzerland and vice versa. Generally speaking though, you should respect the wishes of the authors of the works you are referencing. Robin Hobb, for instance, allows fan art but not fan fiction. A map falls into a sort of limbo between the two, but I suspect that as long as the information contained in a fan-made map does not conflict with what is on the official maps, she would not have a problem with it.

The Eddings' have not posted a policy on fan work that I have seen, but their publisher, Del Rey, apparently have granted permission. (Oh my, David Eddings apparently died last year. I hadn't heard that.)

This is a matter that is somewhat important to me, as I created a map for Robert Jordan's Wheel of Time series (found somewhere on this site), and I remain uncertain about its legality. Jordan authorized MUDs based on his world, but not fan fiction, and his opinion regarding visual interpretations is unknown. Though based on the qualities of the approved cover art, that can't have been too big a deal to him—Rand never looked the same in any of the paintings.