It's salutary that John Bingham, a major crime writer of the 50s and for much of the 60s (and whose career continued for a considerable time thereafter, though with decreasing success) is now unquestionably a forgotten author. I've highlighted his work several times on this blog, focusing on his crime fiction. But he also ventured into spy thrillers - and did so with a considerable advantage, given that he was himself a high-ranking spy.
The Double Agent (1966) is a classic Cold War thriller. It involves a businessman from Yorkshire, Reg Sugden, who is recruited to undertake some low-level spying behind the Iron Curtain, and also to help flush out a traitor in the domestic Secret Services. This novel isn't a fictonalisation of a real life case, but it's impossible not to see a few parallels with the case of Greville Wynne, a businessman and spy who was caught by the Russians and ultimately returned to Britain in exchange for a Russian spy, known here as Gordon Lonsdale.
Bingham was a skilled interrogator, and police interrogations of hapless suspects play a major part in his early books. Here again, the interrogation of Sugden takes up a sizeable chunk of the book. My personal feeling is that this results in a lack of action and pace for part of the story; Bingham, I suspect, realised this, and tried to address the problem, but not entirely successfully. For this reason, the book isn't quite in the class of Len Deighton and John Le Carre, but even so, it's a good read. Julian Symons recommended it strongly, saying that the story is "intellectually and emotionally absorbing because it is so thoroughly authentic."
There are a couple of good twists late in the story, and Bingham's wry observations about spying are always interesting, again in part because he was speaking from experience. He knew that the Cold War was then just the latest in a long series of human conflicts in which cunning and treachery have played a key part. In real life he was much more of an establishment man than either Deighton or Le Carre, but you wouldn't necessarily guess that from this accomplished thriller.