Delete All Suspects — Donna Andrews

After a hit-and-run leaves Eddie, a young techie, in the hospital, his grandmother hires Turing's PI friend Tim to look closely at Eddie's business, which he runs out of their computer-filled basement. So while Turing tries to break into the computers, Tim interrogates the reclusive techie's sleazy friends, who use his computers to run unsavory websites and steal credit card numbers.

But then the feds show up, looking for an online vigilante who's also using Eddie's computers. Caught in the middle, Turing and her friends can't let the vigilante continue, but they can't tell the FBI anything without revealing Turing's identity. So they become vigilantes themselves. And what started as a hit-and-run becomes a deadly game of computer-and-mouse...