Berlin Game by Len Deighton

When a valuable agent behind the Iron Curtain signals he wants out, it's up to Bernard Samson, once active in the field but now anchored to a London desk, to undertake the crucial rescue. But soon, Samson is confronted with evidence that there is a traitor among his colleagues. And to find out who it is, he must sift through layers of lies and follow a web of treachery from London to Berlin until hero and traitor collide.