More like Because It's There

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RiverworldSeries
series37 books1965
Science fiction, about faith and religion, set in far future and other star systems.
“The Riverworld is an artificial, or heavily terraformed, planet where all humans who ever lived throughout history have been resurrected.”
from the catalogue
Start with River of Eternity →
Known SpaceUniverse
universe142 books1964
About aliens, set in 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, Man-Kzin Wars, Fleet of Worlds
Citizen of the GalaxyNovel
Robert A. Heinleinstandalone novel1957
Space opera, about coming of age and identity.
“On a planet far distant from Earth, a lame beggar buys a young boy slave who has forgotten his origins.”
from the catalogue
Tunnel in the SkyNovel
Robert A. Heinleinstandalone novel1955
About survival and space exploration.
“As the final test of a survivalist/explorer class, teenagers are dumped on an alien planet to use their skills while awaiting pickup.”
from the catalogue
Humanx CommonwealthUniverse
universe39 books1972
About aliens, set in space.
“Two species that ought, by every buried instinct, to recoil from one another instead clasp hand to truhand and build a galaxy together.”
from the catalogue
Start with For Love of Mother-Not →
includes Pip & Flinx, Icerigger Trilogy, The Founding of the Commonwealth
Starman JonesNovel
Robert A. Heinleinstandalone novel1953
Space opera, about space exploration, set in space.
“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