Death of a Policeman by M.C. Beaton

Policeman Hamish Macbeth's archnemesis, DCI Blair, wants nothing more than to close Hamish's police station and put him out of a job. Blair sends cop Cyril Sessions up to Lochdubh to spy on Hamish, hoping that Cyril will get some dirt on Macbeth that he can report to Superintendent Daviot. However, Cyril ends up murdered, found on the beach. When vengeful librarian Hetty Dunstable reports to the police that she heard Macbeth say he would shoot Cyril,  Hamish finds himself suspended from work. Hamish, with his usual tenacity and help from his assistant, Dick (and other friends) is determined to uncover who killed Cyril.