I've read quite a bit of RPG-related fiction. Most of it is bloody awful, but FASA put out some very solid Shadowrun books. I certainly didn't get into them because of the game - quite the opposite, I tried the game because the world depicted in the books seemed so captivating. Didn't like the game, though.
Really, a lot of the FASA stuff was good in its heyday. And I did end up liking what is now called Classic Battletech when the Michael Stackpole books got me interested in it.
Like basically everyone else, I find the idea of calling the works of authors like Howard and Lovecraft "RP books" ludicrous. Otherwise, very nearly all famous fantasy and a goodly amount of sci-fi would qualify.
A lot, an awful lot, of the TSR stuff was terrible. Aside from the first Dragonlance Chronicles (which were sloppily written and shakily plotted, but at least solid) and the Dark Elf Trilogy (which really was quite original and decently written, unlike every other Salvatore book I've laid eyes on, gaming or otherwise), it was almost monolithically poor. The Dark Sun and Spelljammer novel series were particularly disappointing - both started with a lot of promise, but TSR fell in love with a round-robin writing system and the various authors never managed to keep a consistent tone from book to book.
The Magic the Gathering novels I've read (not many) were actually a bit better.
None of it is on a level with the likes of Howard or Lovecraft, but then, there's no shortage of awful unlicensed fantasy, either. Frankly, I don't see much difference in quality between the hundreds of derivative unlicensed epic fantasies and the dozens of derivative licensed ones, except that the latter have an excuse.