Well, Norton's Quag Keep is so early in D&D history that it manages to be a D&D novel that feels like it was written by someone who'd heard of the game and written their own version. Also sort of a reverse isekai before that was a popular term, FWIW.
Joel Rosenberg's Guardians of the Flame series has a pretty strong not-D&D-but-close feel in the first few books, then grows into more of its own thing with an anti-slavery message. And it's an actual isekai, proving that their current oversaturation is the product of many decades of insidious growth.
Tales of Talislanta is a short story anthology that does a very good job of feeling like the (quirky) game setting, with the mechanics lurking just off stage. Also a decent homage to Vance's Dying Earth stylistically, much like the RPG.
There are several Traveller novels that probably qualify, with Agent of the Imperium being the most recent I've read. The entire Dumarest of Terra series feels like Traveller as well, unsurprisingly.
I'm probably forgetting something obvious.