Well, 'Brave New World' by Aldous Huxley is a very popular free science fiction book. It presents a dystopian future society which is thought - provoking. 'Do Androids Dream of Electric Sheep?' by Philip K. Dick is another great one. This book inspired the movie 'Blade Runner' and is freely accessible in some e - book libraries. Also, 'I, Robot' by Isaac Asimov, which explores the relationship between humans and robots, can often be found for free.
One popular one is 'The Time Machine' by H.G. Wells. It's a classic in speculative science fiction and is often available for free in e - book form. It takes readers on a journey through time and explores concepts like evolution and the future of humanity.
Well, 'A Wrinkle in Time' is a free popular science fiction. It takes readers on a journey through different dimensions with a young girl as the main character. It's full of imaginative ideas about space and time. Then there's 'I, Robot' by Isaac Asimov. This book introduced the famous Three Laws of Robotics which have influenced a lot of science fiction since. And 'Starship Troopers' by Robert A. Heinlein is also a good one. It has themes of military and space exploration.
I'm not sure which ones are the most popular exactly, but 'Snow Crash' by Neal Stephenson is often highly regarded. It's set in a future where the world has been transformed by a new form of technology and virtual reality, and it has a really unique and engaging plot.
Another popular one might be 'The Uplift War' by David Brin. It's part of a series and in 1989 it was attracting a lot of attention for its unique take on the evolution of intelligent species and the politics between them.
I'm not entirely sure which ones were the most popular, but 'Doomsday Book' was widely talked about. It had an interesting premise of time - traveling historians.
Another popular one could be 'The Uplift War'. It was part of a well - known series and had a lot of fans who were into the idea of the uplift concept, where lower species are 'uplifted' to sentience by higher ones. It had exciting battles and interesting alien species interactions.
'Gateway' is also very popular. Its exploration of human nature in the face of the unknown, along with the unique setting of the alien space station, has attracted a large number of readers. People are drawn to the mystery of the ships and the potential for discovery, as well as the well - developed characters.
Well, 'Ender's Game' by Orson Scott Card is likely to be in the stack. It tells the story of a young boy trained for interstellar warfare. 'Foundation' by Isaac Asimov is another classic. It's set in a vast galactic empire and explores concepts like psychohistory. And 'Brave New World' by Aldous Huxley, which depicts a future society with strict caste systems and technological control over human behavior.
'Wool' by Hugh Howey was quite popular too. It's set in a post - apocalyptic silo, and the mystery and the characters' struggles within that confined space were really engaging for readers. The story gradually unfolds, revealing more about the world outside the silo as it progresses, which kept readers hooked.
Praxis Books science fiction has a good following. They tend to blend different sub - genres of science fiction like space opera and cyberpunk in interesting ways. Their books also usually have well - developed characters that readers can really connect with.