Acceleration by Graham McNamee

Duncan still has nightmares about the day he failed to save a drowning girl. His guilt overwhelms him and his relationships with his family and friends suffer because of it.

Duncan takes a mind-numbingly boring job at the transit authority lost and found. One day, while poking around the dusty shelves looking for a book to read, he comes across a mysterious journal. The journal belongs to a serial killer wannabe who documents the acceleration of his crimes--torturing small animals, setting fires, to stalking women on the subway.

Duncan feels driven to stop the would-be killer before he strikes, but when he tries to turn in the diary, the overworked police are not interested. Duncan, with the help of his two very different best friends, sets out to find the killer.
Plot and characterization are both handled well and the suspense gradually builds to an exciting climax. Very well-done mystery that will keep readers turning pages.

Don't skip the interview with the author at the end of the book. It will give you a fascinating glimpse into the writing of the novel--and it's funny.