Larry Niven has been a mainstay of 'hard' science fiction for over fifty years. Many of his award-winning novels ('Ringworld', 'The Ringworld Engineers', et. al.) and short science fiction (collected in the anthologies 'Tales Of Known Space', 'Flatlander', 'Crashlander') are set in 'Known Space'; our Solar System and galactic space within about fifty light years of it. A select few other science fiction authors, such as Jerry Pournelle, Poul Anderson, Dean Ing, and others have contributed their own take on stories in 'Known Space' via 'The Man-Kzin Wars' series of stories, set against the backdrop of huge interstellar wars between a finally peaceful Mankind struggling to get out of the Solar System with crude STL drives and the millennia old STL empire of the inimical feline Kzinti.
I don't have a total word count on the extent of stories set in 'Known Space', but they fill eleven novels and fifteen (!) 'Man-Kzin War' anthologies. It's one of the largest 'shared universes' in print-only science fiction, and has been one of its most enduring franchises. It even spawned an early RPG ('Larry Niven's RINGWORLD', Chaosium, 1984) which was used as a 'bible' by the authors invited to contribute to the 'Man-Kzin Wars' anthologies. Despite their outward appearance as mere 'space opera', however, stories in Known Space--besides a few whizz-bang hand-waving technologies like FTL drives, stasis fields, and disintegrators--hang together and have stood the test of time; for two main reasons.
First, the characters all come across as quite believable, because they are true to their own species' physiology and psychology. Whether heroes or poltroons, protagonists or antagonists, the characters behave true to their own personalities for very understandable reasons without being cardboard cutouts; and often in ways that subtly subvert the usual tropes long ago established during the 'Golden Age' of science-fiction. Even if the characters are billion-year old telepathic hypnotists ('Slavers') or super-intelligent 'third-stage' humans ('Protectors') or felinoid honor-bound conquerors (the 'Kzinti'), the reader can understand exactly why they do what they do without having to compare them against a 'right' or 'wrong' moral compass. Many of them violate their own species' innate morality for reasons different than any human would, but make perfect sense given what the reader knows about theirs.
Second, the stories set in Known Space are all internally consistent to the degree that the reader is willing to suspend disbelief--and even gloss over minor inconsistencies--due to the sheer inertia of the universe's compelling mythology. Though the universe contains alternately evolved humans, Master Races aroused from billion-year slumbers in stasis, psionically adept humans, aliens of several strange and ultimately dangerous species, and other familiar fixtures of 'space opera' Known Space has been held to a standard of probity and logic missing from many science fiction stories of the past and present--to include several popular beloved TV and film franchises.
In this writer's humble opinion, the only reason Known Space isn't the most popular science-fiction franchise ever is its disheartening lack of video media presence. I advise you to pick up a Larry Niven book and give it a whirl. You'll get lost, and you'll enjoy the trip.