The James Bond novels of Ian Fleming. Fleming is a GREAT writer and those books are some of the best genre pieces ever done. They're very different from the movies -- and for my money, much, much better. Bond is one of the more fascinating characters in literature -- he's insecure and cruel and passionate and romantic and anal-retentive and reckless and a whole mess of contradictions and neuroses that, as Fleming points out, makes him a screwed-up person but a great spy.
Start with Fleming. He's AWESOME.
Go on to Le Carre -- but you only need to worry about three books: The Spy Who Came In From The Cold, Tinker, Tailor, Soldier, Spy and Smiley's People. Le Carre is also a great writer who has an inside track on espionage methodologies and can really show you the petty sorts of motivations and betrayals that go into espionage. Especially good for darker, grimmer sorts of stories, but so full of details on how espionage is actually conducted that they're really unmissable.
Robert Ludlum wrote a lot of good spy-based adventure yarns: The Osterman Weekend is a good one.
Frederick Forsyth's novels: The Dogs of War and The Day of the Jackal are required reading. Forsyth takes you into the unsavoury worlds of paid assassins and mercenary soldiers, and shows you all the details of fake passports, shell companies, sneaking stuff through customs, all sorts of stuff. Really great books.
That'll get you started.
