I'd advise some LeCarre. Then, try reading some of LeCarre's novels. When you've done this, consider checking out some LeCarre. If you like LeCarre, you can't go wrong with his contemporary, LeCarre.
I think you get my point.
Oh, and Graham Greene.
If you don't feel too discriminating, the Mack Bolan series are chockful of things to rip off in your Spycraft games. They're deliriously stupid, full of macho posturing, and loaded with half-assed research...sort of like a Clancy novel, but without the rampant conservatism and much more fun.
Hmmm, but what about LeCarre? (Much more methodical espionage than many... being a spy is often a long, dreadfully dull, job. This is why I liked Smiley's People so much.)
The Auld Grump