The Hill of the Red Fox by Allan Campbell McLean (Part of the Kelpies series)

It is the time of the Cold War. Soviet spies are feared, and secrets are traded. People disappear. Thirteen-year-old Alasdair, living in London, knows nothing of this world. He can’t wait to start his long summer holiday on the Isle of Skye, away from his mother and aunt. But things don’t go quite as planned. On the journey, a stranger gives him a mysterious note before jumping from the train. Worse still, he instantly mistrusts sinister Murdo Beaton, with whom he’s staying. Gradually adjusting to crofting life, Alasdair is not prepared for the web of danger and espionage that unfolds around him. Can he trust anyone?