The Ophiuchi Hotline by John Varley

Used; Hardcover - Good; 1977 1st edition, book club edition; some wear and tear on the dust jacket and some foxing. 

After the effortless capture of Earth by vastly superior aliens, humanity is forced to fight for existence on the Moon and other lumps of airless rock. The invention of the Hotline — a constant stream of data from a star in the constellation Ophiuchus —facilitates survival and enables the development of amazing new technologies. Then, after 400 years, humanity's unknown helpers send a bill for their services...and suddenly everything is threatened once again.

The Ophiuchi Hotline was John Varley's first novel, and it received nominations for both the Hugo and Nebula awards; he later won both for his book Persistence of Vision.