Graham Greene famously divided his writing output into "novels" and "entertainments," with the latter being more light-weight thrillers. The Human Factor, though, is something of a cross-over. It's an espionage novel, but not much of a thriller; instead, this story of a mole in British intelligence focuses on his loneliness and isolation.
It's an enjoyable novel (although very depressing), once I got past the opening setup. The ending images are pretty haunting.