False Witness by Phillip Margolin

 Attorney Karen Wyatt uncovered corruption in the police and the district attorney's office. Then she was framed on fake drug charges and tossed in jail. As this book starts, she gets exonerated and is determined to figure out who in the DA's office is on the take.

Using a large financial settlement from the city, she opened her own law firm. Her current case involves a young mope who was arrested in a stolen car that his friend had told him to drive the friend's girlfriend home in. Unfortunately, the car belonged to a guy who was murdered. And the guy had ties to a dangerous gang. So Karen and the cops are trying to figure out who really murdered Mr Cogen and what he was really up to.  Karen finds she has an ally in the DA's office, a supremely honest lawyer named Muriel, who is trying to help Karen, without putting a target on her own back. It's a compelling story and a quick read.