Joseph Milton was Joseph Hilton Smyth and he started the series with one book "The President's Agent" in 1963 and Joseph Hilton after that Lancer had ghostwriters including the prolific Don Rico and the husband and wife team of Hal Jason Calin and Anne Calin (and possibly others) write the rest of the series under the Milton handle. But Hilton came back once and wrote under the Milton name because paperback publishing of the era was a screwy place. Apparently (if the internet is to be trusted AND its the same guy) in the 40's Hilton Smyth was arrested for being "unregistered agents of the Japanese government" by publishing a pro-Japanese stories in a magazine. So maybe spy-craft was in his veins. He also wrote some novelizations, some stand alone's, including "That French Girl" which got published by Gold Medal, Crest and a some other publishers along the way PLUS some book called "The Sex Probers," which makes me laugh.
Anywho. Bart Gould is a Bruce Wayne type only he doesn't wear the pointy ears instead he spies for the Prez. He gets roped into going to Germany to investigate the disappearances of mild-mannered government employees who hold no vital information, so they know it isn't the Ruskies. Gould was just palying with his grandfather's collection of Derringers so on a whim he packs up his suitcase and loads his trick-sleeve holster with a Williamson .41 single shot Derringer and flies to Vienna. Once there he meets up vile baddies, old flames, ropes in the tried and true reporter friend on the case, drinks, sexes, rents funky European cars, meets a slinky Nazi Femme-Fatale, shoots some dudes, gets his friend killed and goes on a mission of vengeance against a old Nazi dick-head. The climax builds and is particularly nice as it involves a snowed in castle high mountains with a dark history of witchcraft, dungeon escapes, sword fights and derring-doo. The whole package is a slim-wallop of adventure thrills that hearkens back to the classics of the Ruritanian genre like "Prisoner of Zenda"or other swashbucklers and at the same time being 60's modern. A pleasant mixture of Dumas and Fleming.
Gould comes off better then some of the heroes of the day. He's going through the spy-mill for the fun of it and is flippant in the face of grave danger. No dour reflections on the nasty business of spying or dull inter-office politics of intelligence agencies, just plan old rock'em sock'em cliffhanger thrills. The books assumes you have read earlier entries which I had not, Gould's backstory wasn't filled out thoroughly but, eh, I didn't need it. Rich guy = spy is good enough. Gould not being a true agent gives the narrative a lot of wiggle room, the secondary characters aren't secret agents, mostly made up of his friends accumulated over years rich play-boying and he has to act more like a private eye to get his information with no government contacts abroad. It's a nice mix of a lot of pulpy-genres.
This is pretty much what I want when I pick up a spy-fi book. It's not the absolute best iteration of
this genre, not as high as a Malko or a Man from W.A.R. book by Michael Kurland, those sorta of stand lone from the pack. "Baron Sinister" lagged a bit in the early pages but cranked up the juice rather quickly and could have had a bit more of the main villain and made him a bit more dastardly. But those are small quibbles in a book that can be read in a couple of hours. As a guy who's been buying secret agent books since he was a teenager, I think I've had this particular book for like fifteen-years and never read it until now which I can say about WAY too many books. I amassed several other of the series and I look forward to reading more Bart Gould adventures and of course buying the rest of the books. The thrill of the hunt.