Robert L. Forward: Dragon's Egg

As the author himself put it, this is "a textbook on neutron star physics disguised as a novel." 😃

But if that sounds a little too dry, here's a plot summary too:

"Half a million years ago and 50 light-years from Earth, a star in the constellation Draco turns supernova, and the star's remnant becomes a neutron star. The radiation from the explosion causes mutations in many Earth organisms… human astronomers detect the neutron star, call it "Dragon's Egg", and in 2050 they send an expedition to explore it."