Death of an Addict by M.C. Beaton

Are drugs responsible for a sighting of Nessie's long lost relative?
Recovering addict Tommy Jarret has just rented a chalet to check out reports of a sea monster near the village of Drim. But when he turns up dead, apparently from a drug overdose, Lochdubh constable Hamish Macbeth finds the lad's demise to be particularly fishy . . . and not of the local salmon variety.
Deciding to go undercover, Hamish infiltrates the illicit drug trade in nearby Strathbane. But his scheme springs a leak when he's teamed up with a tough Glaswegian detective inspector named Olivia Chater, aka Concrete Knickers. For the lanky lawman investigating drugs and sea monsters it's time to sink or swim - and it would be equally dangerous to flounder . . . or fall in love.
Praise for M.C. Beaton:
'The books are a delight: clever, intricate, sardonic and amazingly true to the real Highlands' Kerry Greenwood
'It's always a special treat to return to Lochdubh' New York Times