"I" is for Innocent (Kinsey Millhone Mystery)

"Smack in the middle of a civil suit, Lonnie Kingman's private
investigator drops dead of a heart attack. Five years ago David Barney
was acquitted of murdering his rich wife. Kingman is trying to divest
Barney of the profits of that murder, and time is running out.
Kinsey Millhone agrees to wrap up Morley Shine's investigation.
So it's a shock when she finds his files in disarray, his informant
less than credible, and his witnesses denying ever speaking with him."