Victor Welles, who's going under the name Jim Cabot, has been called in to take over for another agent who was killed in a car crash. The agent was pursuing a Russian scientist with a head full of secrets, and now Welles has to find him before the Russians get to him. Naturally beautiful women are involved. And menacing tough guys. And lots of chases. The book is very short (like most Ace Doubles), and it moves like an express train. Not to give away too much here, but the ending has a scene of the type the improves any book (and it's not quicksand). I think it should have been extended, and maybe it was cut a bit to fit the Ace format.
The Stark House edition also features Shanghai Flame, Fleischman's first novel for Gold Medal, but the best part is that there's a fine introduction by fellow blogger and longtime pal George Kelley. It's worth getting the book just for that.