Death of a Perfect Wife by M.C. Beaton

Hamish Macbeth is savouring the delights of a Highland summer, but as fast as the rain rolls in from the loch his happy life goes to hell in a handbasket.
The trouble begins when his beloved Priscilla Halburton-Smythe returns to Lochdubh with a new fiancé on her arm. His miseries multiply when clouds of midges descend on the town. And then a paragon of housewife perfection named Trixie Thomas moves into Lochdubh with her cowed husband in tow.
The newcomer quickly convinces the local ladies to embrace low-cholesterol meals, ban alcohol and begin bird-watching. Soon the town's menfolk are up in arms and Macbeth must solve Lochdubh's newest crime - the mysterious poisoning of the perfect wife.