One great humor spy novel is 'Our Man in Havana' by Graham Greene. It's a satirical take on the spy world, filled with humorous misunderstandings and a bumbling main character. Another is 'The Eyre Affair' by Jasper Fforde which combines elements of literary fiction and spy - like adventure in a very comical way. And 'Spies of the Balkans' by Alan Furst also has some humorous moments interspersed among its more serious spy plotlines.
Definitely check out 'The Secret Agent' by Joseph Conrad. While it's not a typical 'humor' spy novel, it has some darkly humorous elements. Another good one is 'Smilla's Sense of Snow' by Peter Høeg. It has elements of a spy thriller and some very dry, Scandinavian - style humor. Also, 'The Spy Who Came in from the Cold' by John le Carré, though more on the serious side, has some moments of wry humor that make it an interesting read for those looking for humor in spy novels.