In reply to Tall Clare:
OK, you have, repeat have, to read Rogue Male. Robert MacFarlane explained why in this Saturday's Grauniad, [available online, but url seems broken] So much follows on from this ... Day of the Jackal for one.
The classic Le Carre Smiley trilogy, where I've always thought Honourable Schoolboy is the best, with the sitauationally trapped protagonist. Pity they never filmed it (yet).
Try Eric Ambler, Mask of Dimitrios. Maybe Buchan, 39 Steps, Greenmantle, to fill in the building blocks of the genre.
Len Deighton, start at the beginning with the unnamed agent (Harry Palmer, from Bolton, insubordinate, Soho in the 60s, so very, very cool) in The Ipcress File and work forwards. I didn't particularly like the later trilogies, too many unconvincing double-bluffs, but each to his/her own. The later Spy Story stands nicely on its own.
These are reading very dated now, cold war - we knew who the enemy was then - but still work as cerebral rather than action pieces, Antony Price's David Audley/Col. Butler sequence. He manages, usually successfully, to place a cold war plot on top of genuine history (WW I, Roman Britain) raising old ghosts. The quality and the action comes in the precision of his dialogue, Oxbridge, civil service, old obligations, as the necessary betrayal goes in over MoD tea in a smoky conference room in Whitehall. He also manages the chronology of the sequence as the lead players become the old dogs, and the young guns are mentored in and tested under fire. Try Other Paths to Glory, or Our Man in Camelot. Old fashioned in the gender relations, but that was the 70s. Mostly on Kindle now, only a few in paperback.
To up the cultural ante: Jospeh Conrad, The Secret Agent.
Post 9/11 spy stories - I haven't come across any great ones yet (yes, I do spend too much time travelling for work, hanging around airport book shops). Unconvinced by Henry Porter's offerings (though his journalistic heart is in the right place). It all moves towards the action stereotype, and you end up hiding a copy of Andy McNab's latest inside the London Review of Books, wondering if you should take up camoflague and off-roading (no!).
Completely off topic, but to show life is stranger - and more moving than fiction - Orlando Figes' Just Send Me Word. True story of a young Moscow couple separated by war in 1941, and then by the Gulag for 10 years. She waited for him.