Top notch reviewer and all-around gamer Jeffro has been writing about the DMG's Appendix N for some time. Instead of reviewing works on Appendix N, I am going to write about books I would add to Appendix N.
I am starting with C.J. Cherryh.
I first encountered Cherryh's works in 1981 when my father got me a copy of Downbelow Station, which I read twice in one month. Downbelow Station was the beginning of her Alliance-Union Universe books, a set of (technically) about 20 books with about half of those in the 'main storyline' of the Alliance and the Union conflicts. The series is interesting for the nature of hyperspace travel (the overwhelming majority of people need to be drugged to psychologically survive jump) and for her analysis of the interplay between different human cultures.
My favorite series of hers is probably the Fortress Series, five books in a decidedly non-Tolkien fantasy world where magic is very, very different than you might think. The viewpoint character might be what is essentially and Elf summoned by a wizard to (once again!) overthrow all the nearby human kingdoms by himself. Or maybe he's just a guy with amnesia. Or maybe he's something else.
A nice, realistic look at what actual fantasy world diplomacy might look like combined with a fascinating concept of magic.
Her first books form the Morgaine Cycle (which are technically in the Alliance-Union Universe) and the books are more fantasy than science fiction (there is no magic, but the 'lost' technology of the gates looks like magic and the worlds involve feudalism, knights, sword fights, etc.). The series is based upon the Gates - devices that allow travel between hundreds, maybe thousands, of worlds. They also permit time travel. At least one major civilization has accidentally wiped itself out by the incautious use of time travel.
The Morgaine of the title is on a quest to destroy all the gates to prevent their misuse and the destruction of everything by time paradox.
Cherryh has actually alien aliens, non-Tolkien fantasy, work based on Slavic folklore, SF westerns where the horses are psionic, and a lot more. Her work is a treasure trove of great ideas for gamers and a fun read on their own.
So - I would add Cherryh to Appendix N