Isaac's Universe is collection of three anthologies in a setting specifically created by Isaac Asimov and edited by Martin H. Greenberg. Isaac, of course, refers to Asimov's first name. Harry Turtledove contributed one short story to each volume.
Asimov's intention was create a shared world from scratch for other authors to use. He did not intend to write any stories but to assist in the development of the universe. The setting was the Milky Way galaxy inspired by Asimov's own Foundation series but also E. E. "Doc" Smith's Lensmen's series in that alien races co-existed with humans.
In this shared world, set about 1000 years in the future, there were about 25 million habitable planets with about 10,000 sapient species but only six high-tech, inter-stellar space-faring species:
- Erthumoi: Human beings, who have colonized many planets beyond Earth and have extended their lifespans with rejuvenation treatments.
- Cephallonians: An aquatic species who live in water-filled spaceships, described by Asimov as vaguely analogous to porpoises.
- Locrians: A skeletal, insect-like species, adapted to a low-oxygen atmosphere with neon rather than nitrogen.
- Naxians: Limbless, snake-like beings, able to read the emotional state of individuals from any organic species just by observing them.
- Crotonites: A small winged species who live in an atmosphere poisonous to the other species, and who consider the other species inferior and look on them with contempt.
- Samians: Physically powerful, slow-thinking, block-like creatures with no appendages who live on a high gravity planet.
These six species live in relative peace since the worlds each is adapted to would require extensive terraforming to be habitable by any of the other five. A seventh, mysterious species, the "Hidden Folk," disappeared from the galaxy millions of years before the first space-faring species came across their artifacts which show no signs of decay or wear. Investigating this species is one of the main plot points of the series.