The Looking Glass War is John le Carré's fifth novel. It was published in 1965.
The novel involves le Carré's signature "Circus" intelligence unit as well as his best-known character, George Smiley — but they aren't the main characters. Instead, we focus on a handful of other British spies who work at an agency known as "the Department," which is slowly being taken over by the Circus.
After a Department mission to photograph alleged Soviet missiles in Germany fails, as does a followup mission to retrieve the film, its operatives solicit a former war agent to head into East Germany to find out what happened. But they have to circumvent the Circus, particularly Smiley and his boss, Control.
The thing with The Looking Glass War is that it's a good book, as le Carré's works tend to be, but it suffers a bit from coming right after The Spy Who Came in from the Cold. Both that novel and its predecessor, Call for the Dead, contain good examples of le Carré's bait-and-switch endings. This one doesn't, at least not to the same extent. It's still worth reading, but readers would benefit from adjusting their expectations accordingly.
It was made into a film (featuring a young Anthony Hopkins) in 1969, and was included as part of the BBC's radio drama series of all of le Carré's Smiley stories. Smiley, not being a substantial character in the book and not yet having benefitted from a more prominent role in Tinker Tailor Soldier Spy, was dropped from the film version.