I'm looking for 1950's era book. US is surrounded by a force field, and a British pilot lands and finds US degenerated due to radiation.
I read this book in my grandparents' house in the late 70's/early 80's, that had been left behind by my uncle, probably in the late 1950's before he went to college. It possibly could have been a little earlier since another book I read then was the hardback Hal Clements' A Mission of Gravity that was published in 1954.
The book I am trying to find was a paperback containing two novels (novellas?) with the cover art on both sides of the book and to read it you would flip it over. I don't remember the publisher.
The plot is that after WWII the US had the atomic power and the bomb, and isolated themselves from the rest of the world and technologically was thought to be much more advanced. The British RAF pilot is doing reconnaissance flights notices the barrier is down and flies in and lands. There he eventually finds many technologically wondrous things such as an underground transportation system connecting the major cities that can transport him across the country. Eventually he ends up in Washington, D.C. The story becomes about how the dependence on atomic power leads to the degeneration of the people. I am fuzzy about how that worked but there was maybe devolution or loss of intelligence or loss of fertility. Hence one of the elite deciding to lower the barrier and let the British pilot land and explore with options of restarting interactions with the rest of the world. I think the time was sometime after 2000, but not dramatically so.
I guess there's a chance the that the pilot was Canadian instead of British, and I don't remember exactly if it was just the United States, or all of North America that had the force field.