For someone who enjoys both SF and gaming, "Dream Park" is a no-brainer favorite. A D&D game (LARP, actually), wrapped in a fantasy, wrapped in a mystery, wrapped in SF, it's the first and best of the Nivens/Barnes DP franchise books. It focuses on the individual characters (and the contrast or reinformcement of, for the players, their fantasy overlays), and is, in many ways, the quintessential RPGing novel.

Though getting long in the tooth now, only the occasional odd anachronism pops up (fanfold paper being a big one). Worth reading and rereading again.