More like Critical Threshold

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Known SpaceUniverse
universe142 books1964
Science fiction, about aliens, set in far future and space.
“Give a clever species a few centuries and a working fusion drive, and it will fill the sky with problems.”
from the catalogue
Start with Ringworld →
includes Ringworld, Fleet of Worlds, Man-Kzin Wars
HeecheeSeries
series19 books1972
completed
Space opera.
“The Heechee are an advanced alien race that visited the Solar System hundreds of millennia ago and then mysteriously disappeared.”
from the catalogue
Start with Gateway →
The Colors of SpaceNovel
Marion Zimmer Bradleystandalone novel1963
Space opera, mysterious in tone.
“The Lhari seem peaceful and friendly, but they go to some lengths to protect their monopoly on faster-than-light travel.”
from the catalogue
Starman JonesNovel
Robert A. Heinleinstandalone novel1953
Space opera, about coming of age, set in spaceship.
“Starman Jones, a 1953 science fiction novel by Robert A. Heinlein, features a farm boy who wants to go to the stars.”
from the catalogue
CoDominiumUniverse
universe98 books1971
Military science fiction.
“The body of work runs along a few hard threads rather than one line.”
from the catalogue
Start with The Mote in God's Eye →
includes Moties, Falkenberg's Legion, Laurie Jo Hansen, War World
UpliftUniverse
universe13 books1980
Space opera, about uplift.
“No species, the Galactics insist, ever climbs to sapience alone.”
from the catalogue
Start with Sundiver →
includes Uplift, Uplift Storm