The absolute best espionage writer that nobody ever reads is Adam Hall (Elleston Trevor). His Quiller series (19 books) is probably the best espionage fiction I've ever read. The writing style can take a bit to get used to, but absolutely nobody is as good at creating suspense with the written word as Trevor.
Vince Flynn is the real deal, although from time-to-time, he can get a bit preachy and that gets tiresome.
To my mind, the best espionage/thriller writer around today is Barry Eisler. His John Rain series (6 books) is absolutely terrific.
Some of David Morrell's books are certainly espionage and, except for Adam Hall, nobody is as good at putting suspense on a written page as Morrell. His trilogy (The Brotherhood of the Rose, The Fraternity of the Stone, and The League of Night and Fog) is not to be missed. First Blood (the novel upon which the first Rambo movie is based) is also an excellent read (and the characters are much more "real" and less cardboard and the story itself is much more a case of shades of grey). You can rarely go wrong with Morrell.
For a bit of "old school" espionage, do yourself a favor and track down some of Charles McCarry's early novels.
Though a bit dated now, Trevanian had several excellent espionage stories (The Eiger Sanction, The Loo Sanction, and Shibumi).
Among new authors, I'd recommend Brett Battles and David Stone.