The Spy Who Came in from the Cold by John LeCarré
He is duly recruited as a defector, promised money, and meets his contact in Holland to spill the beans. But his contact soon lures him into the East where the contact is accused of treachery and Leamus finally realizes his real role in the mission. And when the girl turns up--having been "given a trip east a reward" by her local Communist party--Leamus.
LeCarre's message is that human treachery is human treachery whether out of personal cussedness or in the name of the State. And one side is no better than the other. Leamas, about as far away from an idealistic character as one can imagine, rebels.
George Smiley is a more important character in this one than I remembered and not as likable as he becomes later. He's clearly the brains behind the London plot. (Very interesting in these early novels by LeCarre to see how Smiley must have grown on his creator.)
Now I've got to watch the film again--I love Richard Burton in the part. Couldn't resist uploading the movie cover, not the book cover.