Spy Sam Durell goes undercover to root out a traitor, though through the machinations of a fiery redhead (is there any other kind?) is branded a traitor himself in Edward S. Aarons' Assignment Treason.
I have found that Aarons' work is far above the usual Gold Medal pulp fare and reads even better upon rediscovery. I would compare him favorably to Donald Hamilton (and his Matt Helm novels) of roughly the same period; hard-nosed writing and surprisingly politically nuanced plots.
This one, written in the late 50s, features a red-baiting Texas senator in the Joe McCarthy vein with a skewed plan to nuke away detente before the U.S. gets too soft. The paperback sports one of those memorable Gold Medal covers, with Sam Durell fishing a nude woman out of the surf (which to their credit actually happens in the novel as an angry, obese naked woman attacks them both--you just have to read it). But the content inside is quite solid throughout.
I bought this in a giant ebay lot of Gold Medal books featuring Aarons and Philip Atlee and others and have been working my way through the enjoyable stack.