The best D&D novels of all time... are not officially D&D novels.
Villains by Necessity, by Eve Forward is set in a world in which Good has won. And what results from that. I have not seen a statement by Ms. Forward that this was built on D&D, but I find it difficult to think otherwise, as the character classes are pretty darned clear. Unfortunately, this one is currently out of print.
The Deed of Paksenarrion, by Elizabeth Moon, is the story of a paladin coming into her powers, and her career. It is a trilogy (Sheepfamer's Daughter, Divided Allegiance, and Oath of Gold), and I'd recommend it to just about anyone interested in D&D novels.
Grunts by Mary Gentle... isn't gentle. It includes the line, "Pass me another elf, Sergeant. Mine's split." It isn't really about D&D. It is about orcs. But orcs are either Tolkien or D&D. And these orcs get assault rifles. So.. it ain't Tolkien. It also has what I feel is the absolute best cheesy opening sentence ever written in the fantasy genre.