I think '2001: A Space Odyssey' by Arthur C. Clarke is very influential. It has inspired many movies and other works. 'Fahrenheit 451' by Ray Bradbury is also important as it warns about the dangers of censorship in a future society. And 'The War of the Worlds' by H.G. Wells is an early and very influential work about alien invasion.
One of the best hard science fiction books is 'The Martian' by Andy Weir. It's about an astronaut stranded on Mars and his struggle to survive using his scientific knowledge. The story is filled with accurate scientific details about space travel, botany, and engineering.
Well, 'Ender's Game' by Orson Scott Card is a must - read. It's about a young boy trained in military strategy in a future world. 'Foundation' by Isaac Asimov is also excellent. It deals with a galactic empire's decline and the attempts to preserve knowledge. And then there's 'Brave New World' by Aldous Huxley, which explores a society where people are genetically engineered and conditioned for specific roles.
One of the most recommended science fiction books is 'Dune' by Frank Herbert. It has a richly detailed universe with complex politics, ecology, and technology. The story follows Paul Atreides as he navigates a dangerous desert planet filled with giant sandworms and feuding noble houses.
They are indeed interchangeable. The term 'sci fi' has become so commonly used that it has the same meaning as 'science fiction' in most contexts. When you think about it, they both describe a type of fictional work that involves elements of science, technology, and often the exploration of new worlds or ideas. Whether it's a 'sci fi' novel or a 'science fiction' TV show, we expect to see things like spaceships, robots, or strange new scientific discoveries.
I would recommend 'Rendezvous with Rama' by Arthur C. Clarke. It's a story about an alien spacecraft that enters our solar system. Clarke is known for his hard science in his works, and this book is full of scientific speculations about the nature of the alien object. 'Hyperion' by Dan Simmons also stands out. It has a complex alien civilization called the Shrike. The book combines elements of hard science fiction with deep philosophical questions about the nature of these alien beings and their relationship with humans.
A good military sci - fi book is 'Old Man's War' by John Scalzi. It features an old man who enlists in the military to fight in an interstellar war. The book has great descriptions of military tactics and alien species. 'Halo: The Fall of Reach' is also a great choice for fans of military sci - fi. It is set in the Halo universe and details the origin of the Spartan - II super soldiers. Another is 'Warhammer 40,000: Horus Heresy' series which has a vast military - like setting with different factions, complex politics and intense battles in a far - future, dark and gritty universe.