You can download 29,000 free ebooks in 25 different formats for compatibility with your ereader (including some audiobooks) at ManyBooks.net. Here are some of their science fiction novels that I can recommend.
by Edgar Rice Burroughs (1912)
This is the first book in the ten-book Barsoom series written by the creator of Tarzan. All of the books in this series are also available as audiobooks.
John Carter, a Civil War soldier, falls asleep in a cave and wakes up on a war-torn Mars. He must learn the local cultures and languages and choose a side in the battle between native races to control Mars. (All ten books in the series are available).
by Rudy Rucker (1982-2000)
Four novels: Software, Wetware, Freeware and Realware. The series of novels features: robots, artificial intelligence, viral intelligent fungi, bio-augmentation and super evolution. I know of nothing comparable to this series as far as way-out extrapolation is concerned.
by Jules Verne (1874)
A sequel to Verne’s 20,000 Leagues Under the Sea. Also available as an audiobook.
Soldiers escape a Civil War prison camp by stealing a balloon, which is carried by storm to a remote volcanic island. Surviving on the island is complicated by giant plants and animals as well as blood-thirsty pirates and a dangerous volcano.
by Olaf Stapledon (1937)
The book describes the past and future of human life over a vast 2 billion year span and across the universe. Breaking away from contemporary thinking, it hosts many imaginative and philosophical ideas of the far future.
by H.G. Wells (1901)
Two men, one an eccentric scientist, construct a space vessel and use it to travel to the Moon. Unlike popular Lunar fantasies, Wells gives his best scientific picture of the journey to and exploration of the Moon.
(The Time Machine, The Invisible Man and The War of the Worlds are also available)
by Richard Kadrey (1988)
One of the few post-1930 selections available. A cyberpunk novel in a future Los Angeles plagued by hunger and disease.