Ophiuchi Hotline
From the back cover:

The invaders came in 2050... They did not kill anyone outright.  They said they came on behalf of the intelligent species of Earth -- dolphins and whales.  The invaders quietly destroyed every evidence of technology, then peacefully departed, leaving behind plowed ground and sprouting seeds.  In the next two years, ten billion humans starved to death.

The remnants of humanity that survived relocated to the moon and other planets.  But they are not alone in their struggle -- someone or something, somewhere in deep space, is sending them advanced scientific data via the Ophiuchi Hotline.  And by the twenty-fifth century, the technological gifts from the Hotline -- especially its biological and medical solutions -- have created a world unlike any ever known or imagined...
Science fiction, Fiction, general