This does not mean that it is unregarded. It's not too hard to find as a collectable if you have a couple of hundred dollars to spare. But Rosana Hart, Paul Lineberger's daughter recognises that there are plenty of people who don't want to collect it, they just want to read it, and so she has made it available as a PDF. Which is how I got to see it.
It is 1949. Scraps of intelligence taken together hint at a secret underground russian city where research is being done into atomic weapons. It is known as Atomsk. Major Michael Dugan is given the task of infiltrating Atomsk, finding out what is going on there, and then leaving enough of a mark behind him so that the russians know they have been infiltrated and Atomsk is no longer an ace up their sleeve.
We follow Dugan as he slips through Russia, assuming and dropping identities as required. Unlike so many fictional spies, Dugan has to work his way to the secret city one step at a time, using skill and cunning. What would be an afternoon jaunt for Sydney Bristow is a journey that takes weeks for Dugan.
The weakest point to me was when we finally reach Atomsk and don't get to see much of it. It's not necessary to the plot, but after the big buildup I would have liked a little more sightseeing of the big mysterious place we've heard so much about.
The book is clearly an earlier work than Lineberger's science fiction. The lyrical writing is there, but it is not as developed as his work as Cordwainer Smith. The really striking thing about Atomsk is how optimistic it is. Dugan is such a good spy because he empathises with the people he encounters. He believes that the work he is doing is not just for the good of his own country, but for its enemies too. During World War 2 he works undercover in Japan as an incompetent officer gently fouling up every major project that comes through his department, saving lives not just of Americans, but Japanese too.
The book has also aged remarkably well. I mean obviously it is now a period piece, where it was "present day" when it was published, but there is nothing out of place or jarring to the modern reader, which suggests how far ahead of his time Linebarger was in his attitudes.
Atomsk is not the great american novel, but it's a fun spy story that proves that realism and optimism can exist together. There are a lot of writers today who could learn from this.