This is, I believe, the third John LeCarré I've read, after The Spy Who Came In From the Cold in high school and The Little Drummer Girl in college. The Looking Glass War can be considered "minor" next to those giants of the genre--but like his literary forebear, Graham Greene, even LeCarré's lesser novels are worth reading.
What I liked best about Looking Glass War was its perspective--it's set among the agents of Britain's military intelligence, the Department, rather than the cover-but-civilian peacetime operators of "the Circus." Twenty years after WWII, the Department finds itself lowest on the totem pole--marginalized and unsupported. They don't even have a motor pool. So when an East German comes to them in Hamburg claiming a Soviet missile installation nearby, Department head LeClerc jumped at the chance to recapture old glories, despite the information being's less than convincing. They recruit a naturalized Pole, Fred Leiser, to cross the border and confirm the story.
Like the other books of his I've read, Looking Glass War is less a collection of action setpieces--you know, like every spy movie ever--than it is an examination of the relationships between spies, the pressures of bureaucracy, and the toll on the human psyche of constant deception. And while this is why he (and Greene) tend to get shelved in Serious Literature, these are traits that should be found in all thrillers. Emotional investment, no matter what the genre, elevates an OK book to a great one.