The Fregoli Delusion - Donaghue and Stainer Crime Novels, #3

The third Donaghue and Stainer Crime Novel. When billionaire H. J. Jarrett is shot to death on a bike path in prestigious Granger Park, the killer is seen running away by a man who is apparently in the right place at the right time. However, Lieutenant Hank Donaghue and Detective Karen Stainer discover to their dismay that their only eyewitness suffers from a rare psychotic disorder that makes his testimony useless.
As Donaghue's investigation focuses on the top one percent of Glendale's social stratum, including close friends of his own mother, Stainer findsherself alone when her gut instinct tells her that their eyewitness is right after all!