Are there instances of someone who wrote a successful book out of adventures played with friends?
I'm not sure if these qualify, but here are a few examples:
-Rob Kuntz of Pied Piper Publishing has publishing modules such as the Living Room based on his Game Master experiences from the early days of Dungeons and Dragons.
-The hit anime/novels Records of Lodoss War actual began as an actual play recounting of a RPG session in Japan.
-Ed Greenwood spawned countless novels and articles of the Forgotten Realms based on stories he had been writing (and later playing) since he was a child.
-Gary Gygax of course wrote the Gord the Rogue novels, with many of their roots from Gary's own D&D world!
So yes, there's always hope!
Also my current favorite fantasy series Malazan Book of the Fallen by Stephen Erickson started out as a GURPS fantasy campaign he had.
I don't know if it counts, but Dominic Deegan was inspired by a D&D session, where the creator played a Divination Wizard.