The Last Detective (Peter Diamond #1)

A nude female corpse has been found floating in a large reservoir just south of Bristol.  In order to solve the mystery of the "Lady in the Lake,"  Detective Superintendent Peter Diamond must locate two missing letters attributed to Jane Austen and defy his superiors on the force to save a woman unjustly accused of murder.  This is the first of the Peter Diamond series; it won the 1992 Anthony Boucher Award for Best Mystery Novel.