Hook or Crook — Gerald Hammond

In this latest novel in Gerald Hammond's acclaimed Keith Calder mystery series, it is Calder's friend Wallace James who plays the (at first) reluctant sleuth.
James, a master fisherman, and his young pupil Eric Bell are fishing for salmon when they trip over the body of a man lying face-down in the river, a fish hook embedded in his cheek. What at first appears to be an accidental death starts looking fishier when Wallace James and the police cast their nets for clues.
To their confusion, the dead man turns out to have had suspicious dealings in the Middle East, bringing to the wild Scottish Highlands a sinister international element - and another confounding mystery to be untangled.