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!
Tracy Hickman has indicated on several occasions that much of the Dragonlance content was inspired by his own experiences playing with Margaret Weis and several other (at the time) TSR employees.
Also my current favorite fantasy series Malazan Book of the Fallen by Stephen Erickson started out as a GURPS fantasy campaign he had.
Yes Raymond Feist and his Midkemia/Riftwar series was born out of an early roleplaying campaign. Probably one of the first.
I don't know if it counts, but Dominic Deegan was inspired by a D&D session, where the creator played a Divination Wizard.