How can it be that Donald Hamilton's Matt Helm books are out of print? Maybe it's just a matter of money, with his estate asking too much for reprint rights. I can't think of another reason because this is one of the best spy series ever. In fact, I'd put it right at the top.
Death of a Citizen is the first book in the series. Matt Helm is a photographer and writer, living peacefully in Santa Fe, long past the days in WWII when he was a member of a super-secret government agency dedicated to things like assassination. Mac, the head of the agency, told Helm after the war that what they did "never happened."
But we all know that the past is never dead and that secrets are never buried deeply enough. And that super-secret agencies never shut down. An operative named Tina comes back into Helm's life and with her the violence that Helm thought he'd turned his back on. There's a body in a bathtub, and Helm's daughter is kidnapped. Here's a tip: You should never kidnap the daughter of someone once code-named Eric. Helm begins to revert to his old ways, and before long the citizen Helm is dead and the operative Eric is back.
This is hardboiled fiction at its best. The way Helm handles the kidnappers, and especially his final disposition of them, is still shocking even today, though its impact was much greater when the book was published 50 years ago. If you've never read Hamilton's work, particularly this series, Death of a Citizen is the place to start. I can almost guarantee you won't stop until you've read more.