A lot of older books are in the public domain now and Project Gutenberg has a lot of those available in various formats for downloading to your computer. I've previously downloaded some of the Doc Savage and The Shadow books, and a friend of mine recently pointed out a page where they have a bunch of free SF/Fantasy books, many of which took me on a nostalgic spin back to my childhood. In particular, I remember fondly the books by Edgar Rice Burroughs and the book Gulliver of Mars, which was an influence on ERB. I sometimes download these books even if I have a hard copy because it makes it easy to use my computer to search out particular phrases of interest. I've used this feature a few times for articles that I've done and found it handy.
Now, it's off to download a few more old books. I've particularly got my eye on Warlord of Kor by Terry Carr, and Brigands of the Moon by Ray Cummings. I've never read either one of them and but those titles are a promise I can't pass up.