[FiatLuxDave]

I'd choose one of these two, since they both completely qualify as underrated:

A) "Psychohistorical Crisis" by Donald Kingsbury, which is an unauthorized sequel set in the world of Asimov's Foundation Series.

B) "Venus Equilateral" by George O. Smith, which is an old, rather cheesy sci-fi novel from the 1940s, which is basically a bunch of engineers on a communications space station fighting lawyers, bureaucracy, and space pirates through inventing stuff.