There are a lot of good books to read for free at Project Gutenberg. According to their home page, they have over 53,000 titles which can be read in a variety of formats.
I have read many science fiction books there that are now out of copyright. A recent read of mine is mentioned in my current Odd Language item: Odd Language #188: Proving for Whom?, namely, Andre Norton’s The Time Traders.
Have a look, and let me know if you find any gems.