"BioShock: Rapture is a written work set in the eponymous BioShock universe. John Shirley is the author, and the publisher is Tor Books. It's story covers events from the creation of Rapture until a certain point before the first game. Both familiar and lesser-known characters from the BioShock universe will be expanded upon in the story, as well as locations like Apollo Square and Point Prometheus. "
Take a gander at the BioShock Wiki and you'll get more information on it. There's also some really, really nice cover art pieces floating around. Looks really promising and I'm already sold on it.
(Thanks to Kyburz on the official BioShock forums for the informative post.)