Some good long science fiction books are 'The Lord of the Rings' by J.R.R. Tolkien, although it's more of a fantasy - science fiction blend. It has an elaborate world filled with different races, magic, and a great battle against evil. 'Ender's Game' and its sequels by Orson Scott Card are also great. The story follows a young boy, Ender, in a military training program against an alien threat. The sequels expand on the universe and the consequences of Ender's actions.
I would recommend 'Neuromancer' by William Gibson. It's a cyberpunk classic that is quite long and delves deep into a future where technology and humanity are intertwined in complex ways. Also, 'Snow Crash' by Neal Stephenson is a long and engaging read. It has a unique take on a future society where the virtual world and the real world collide, with a fast - paced plot full of action and interesting characters.
One great long science fiction book is 'Dune' by Frank Herbert. It's a complex and immersive story set in a far - future universe with detailed world - building, political intrigue, and a hero's journey. Another is 'Foundation' by Isaac Asimov. This series spans multiple books and explores the fall and rise of galactic empires through the science of psychohistory. 'Hyperion' by Dan Simmons is also a long and excellent choice. It weaves together multiple storylines of different characters on a pilgrimage to a mysterious Time Tombs on the planet Hyperion.