My Frail Blood — Gavin MacDonald

DCI Forsyth and DS MacRae skive off from a course they are attending in Inverness to investigate the murder of the alcoholic wife of a professor. She appears to have been knifed after returning to their house up a glen from a drinking binge, her body being dumped in a nearby loch. When ome of her lovers is shot dead with an arrow in the back after attempting to blackmail the murderer, Forsyth presents his former assistant, who is in charge of the case, with the solution, arrived at by logical deduction.