John Buchan 
Island of Sheep 

In the final instalment of the Richard Hannay series, the popular secret-agent goes on his most dangerous mission yet. Now retired and in his fifties, Hannay is bound by an oath to protect the son of an acquaintance who is about to inherit the secret to a priceless treasure. Unfortunately keeping the young man, and his secret, safe proves difficult as a number unscrupulous individuals all want in on the treasure, including some former adversaries of Hannays. This time Hannay is joined by his own son, Peter John, and some old allies in this final, exciting adventure that ends Buchans popular series on a high note.