Old Sins - Lindenshaw Mysteries, #4

Detective Chief Inspector Robin Bright and his partner, deputy headteacher Adam Matthews, have just consigned their summer holiday to the photo album. It's time to get back to the daily grind, and the biggest problem they're expecting to face: their wedding plans. Then fate strikes-literally-with a bang. Someone letting loose shots on the common, a murder designed to look like a suicide, and the return of a teacher who made Robin's childhood hell all conspire to turn this into one of his trickiest cases yet.
Especially when somebody might be targeting their Newfoundland, Campbell. Robin is used to his and Adam's lives being in danger, but this takes the-dog-biscuit.