What a tough one! Most of the novels that I read that are licensed aren't from RPG's but rather TV/movies (e.g. Star Trek), or wargaming (Dan Abnett's 40K novels). Aside from D&D novels, what's out there? I know there were some World of Darkness novels, but they were just flotsam in the blight of vampire literature that afflicted the earth.
Oooh! Oooh! I know!
The D&D "not Choose Your Own Adventure" novels! I loved these so much I recently bought them for my kids to read. They're fun.