A Certain Slant of Light

Call for the Dead is mainly interesting as a kind of prelude to his later work -- the first exploration of "his subject, his idea, what the French call his donnée" as James puts it. Smiley being brought in to investigate, Smiley being framed, Anne's promiscuity, the former victim of the Nazis who becomes fearsome in East Germany, Mendel and Guillem, Smiley as Holmes rather than Bond, the twist and counter-twist near the end....so much is there, already developed, including the bleak general outlook and ambiguous victory. But it doesn't quite all jell, not yet.

Just as you say, it starts off more as a murder mystery, but almost as disguise; Murder of Quality is much more a murder mystery with spies in, not a spy story misleadingly centered around a murder. (Murder of Quality also has twists that remind me very much of Christie!) The Spy Who Came in from the Cold is really where you get the first full horseradish experience, I think. It shows perhaps his coldest view of humanity (interestingly, he revisited it in a retelling by Guillem, over fifty years later!). The movie is also excellent, and keeps the terribly depressing ending.

It kind of fascinates me that then, after his first big triumph re his chosen subject, artistically and professionally, he backs away in the next three novels from postwar Britain, then Smiley, then espionage itself until he writes the truly terrible autobiographical story of his sex life, which was justly panned. So he returned to the Circus, as Smiley always does, for his most famous books.