Two novels for the price of one. The second may be a little weaker than the first, but Larry Niven does not produce any bad writing. In common with the rest of his work, these contain rock hard science. Take a neutron star, put a planet round it and you have the recipe for a gas torus — a vast volume of micro-gravity with breathable air. These books bring orbital mechanics to life, if you notice it amidst the riveting story.
The Integral Trees