Thorndike Press large print core series

"Berlin, 1963. The height of the Cold War. An early morning spy swap at a discreet border crossing. The Communists are trading two English students caught helping friends over the wall and an aging MI6 operative. On the other side of the trade: Martin Keller, a physicist who disappeared into the English prison system. Keller's most critical possession: his American passport. Keller's most ardent desire: to see his ex-wife Sabine and their young son. But he has questions: who asked for him? The KGB? He has worked for the service long enough to know they want him for something. Something which he cannot learn until he arrives in East Berlin, when suddenly the game is afoot"--