K is for Killer by Sue Grafton

When Kinsey Millhone answers her office door late one night, she lets in more darkness than she realizes. Janice Kepler is a grieving mother who can't leave the death of her beautiful daughter Lorna alone. The police agree that Lorna was murdered, but a suspect was never apprehended and the trail is now ten months cold.

Kinsey pieces together Lorna's young life: a dull day job a the local water treatment plant spiced by sidelines in prostitution and pornography. She tangles with Lorna's friends: a local late-night DJ, a sweet, funny teenaged hooker, Lorna's sloppy landlord and his exotic wife. But to find out which one, if any, turned killer, Kinsey will have to inhabit a netherworld from which she may never return.

Now it's your turn to join Kinsey Millhone as she looks for a killer.