Second Chair by John Lescroart

A teenage boy named Andrew is arrested for killing his girlfriend and their drama teacher. All the evidence points to his guilt but he insists he didn't do it. Young lawyer Amy Wu, still reeling from the death of her father months earlier and drinking heavily, gets the case. At first she's convinced he's guilty and just tries to cut him the best deal possible. But after awhile, she's not so sure he really is guilty. Her boss, Dismas Hardy, worried about Amy's mental state, decides to join the legal team as second chair. And soon the two are frantically trying to find evidence to exonerate their increasingly depressed client. At the same time, Dismas's best friend, high-ranking cop Abe Glitsky, is dealing with a series of murders of ordinary citizens--that may be the work of the same professional killer.

As always, John Lescroart tells a compelling story with interesting characters. Once you get hooked, you won't be able to put it down.