Appendix N is the list of "inspirational" fiction provided in the back of the 1979 Advanced Dungeon and Dragons Dungeons Masters Guide. It list such esteemed authors as J. R. Tolkien, Jack Vance, Edgar Rice Burroughs, Robert E. Howard, and H.P. Lovecraft and lesser known but popular authors such as August Derleth, Lord Dunsany, Abraham Merrit, Manly Wade Wellman and a few dozen more. Some authors include suggested books and series next to their names and others are just listed. Given the ambiguity, what counts as an Appendix N book is a bit flexible.
It's become a bit like a D&D cannon for the OSR community who regard it as the foundational text behind their favorite hobby. It's not hard to see why. Gygax et al may have shown an enormous amount of creativity in the synthesis of ideas but the number of wholly original notions is small. The alignment system is taken from Michael Moorcock's Elric saga, the D&D troll is a perfect match for the troll in Poul Anderson's Three Hearts and Three Lions in both powers and appearance, and the Prismatic Spray spell is right there in Jack Vance's Dying Earth. Despite being a soup of not even concealed plagiarism/homage the only real trouble they got into was for using the word Hobbit which they changed to halfling and which has since been shown not to be original to Tolkien anyway.
Endless articles have circulated in the OSR blogosphere about the list in general and books and series in specific. It's spawned two podcasts: Appendix N Book Club for general examination, and Sanctum Secorum which focuses on the Dungeon Crawl Classic RPG and its relation to Appendix N. Of course if you want the true essence you'll need to hit the used book stores. While much of Appendix N is in the public domain the majority of it is out of print and while most of it can be found on Kindle or Audible if you want the genuine experience it should be from cheap, yellowing pages in dime store paperbacks.
IRON NODER: WE'LL RUST WHEN WE'RE DEAD