Echo Park by Michael Connelly

Just as he's on the verge of cracking an unsolved case, Detective Harry Bosch discovers an old clue that could have saved lives -- and the guilt begins to haunt him.
In 1993 Marie Gesto disappeared after walking out of a supermarket. Harry Bosch worked the case but couldn't crack it, and the twenty-two-year-old was never found. Now, more than a decade later, with the Gesto file still on his desk, Bosch gets a call from the District Attorney.

A man accused of two heinous murders is willing to come clean about several others, including the killing of Marie Gesto. Taking the confession of the man he has sought -- and hated -- for thirteen years is bad enough. Discovering that he missed a clue back in 1993 that could have stopped nine other murders may just be the straw that breaks Harry Bosch.