Dark Sacred Night by Michael Connelly

When LAPD detective Renee Ballard finds Harry Bosch going through a colleague's file drawer, she learns that he's privately looking into the murder nine years ago of a teenager named Daisy Clayton. Since the crime is still unsolved, Bosch is investigating as a favor to Daisy's mother, Elizabeth. Ballard is intrigued by the case and agrees to help Bosch go through all the old material and also look at other crimes and criminal activity around that time. Ballard is also busy with other cases that arise as she works the night shift in Los Angeles, while Bosch finds himself entangled with a local gang. Dark Sacred Night showcases the Ballard/Bosch partnership, which looks like it might turn into a series of its own. It will be published in October.