Dragon's Egg [1] and Starquake [2] by Robert L. Forward are both hard sci-fi novels about fast-moving life on a neutron star. I enjoyed both of them, especially the xenobiology and xenoarcheology.
Flux [1] by Stephen Baxter is also about the inhabitants of a neutron star. Hard sci-fi, but maybe not quite as Hard as Dragon's Egg so far as I recall.
[1] https://en.m.wikipedia.org/wiki/Flux_(novel)