This is a great topic; and an interesting tension whether to recommend thing you like, or things that you think will improve one's understanding or be applicable to your D&D game?
For direct applicability:
First off...pretty much everything on the original Appendix N, with special emphasis on Poul Anderson--who after Jack Vance is probably the author with the most direct influence on the cosmology of D&D.
Fafhrd & the Gray Mouser for how to introduce a little humor and mild satire, and create interesting bite sized one night adventures.
I'll add Leigh Brackett's Eric John Stark stories to see how to combine pulp style bare chested barbarians with scifi.
The Horseclans stories by Robert Adams for great medieval combat and tactics, as well as a great fictional depiction of logistics. Keep in mind there is some severe values dissonance between the books and modern values. If that's important to you, steer clear.
It's scifi, but the Dumarest of Terra books are a master class in how to create new and vibrant worlds, and attendant challenges with just a few brush strokes. Plus, Earl Dumarest is a great PC template--simple motivation which justifies getting in all sorts of adventures. Plus, as a Traveller fan, it's fun to see how much of a literary precedent this series is to that game.
I'll probably think of some others before too long.
