I just finished This Is Not a Game by Walter Jon Williams.

This book is built around alternate reality games, where the term "This Is Not a Game" is used to keep players and game runners from doing things that would undermine the real feeling of the game.  It involves four people who met through gaming.  The protagonist creates and runs alternate reality games for a company owner.  A third character is an investor and a fourth is an ex-lover.

It starts off with the producer protagonist getting stranded in Jakarta during a crisis and then using her game world connections to help her get out.  That introduces the themes of games vs. reality and using the game players to effect real world events.  In the second part, this gets developed more as another character is murdered and the line between reality and the game blurs.  This kind of game/reality blurring isn't uncommon (see David Fincher's The Game for a film version of this), but the twist here is that the protagonist always knows what is game and what isn't - it's only the players who don't.

Overall, a good read.  Kept me interested enough to finish it in a day or so.