The Scorpion Signal — Adam Hall

An older, cynical, and embittered Quiller is running on empty. He needs his vacation badly, but is suddenly sent on an urgent mission to Moscow. Schrenk, a British agent and an old partner of Quiller''s, has been captured and tortured by the Russians in Lubyanka Prison. Schrenk has now escaped but has disappeared, making no contact with his superiors in London. Quiller''s mission is to find him.