C is for Corpse by Sue Grafton

Kinsey is hired by Bobby Callahan because, nine months ago, someone tried to kill him by forcing his car off a bridge. Bobby survived, but his friend and passenger, Rick, was killed. Bobby had major injuries and memory loss after the crash, but is convinced that the person who wanted him dead is still after him. Bobby has no idea of who it might be, but is hoping that Kinsey can figure it out. Then, three days later, Bobby dies in another car accident. Kinsey, though, is determined to bring the culprit to justice.