Project Gutenberg allows you to download free ebooks that can be viewed on your computer or added to a digital reading device like Kindle. The content is totally legit because the site only offers books that have an expired US copywrite (unless otherwise noted), which means you can finally download Leviathan guilt free.
If you'd prefer something a little newer than Gutenberg can offer, try the Digital Media Locator.
The Digital Media Locator is a directory of local libraries - which means can you check ahead of time to see if that new Gideon Defoe ebook is available instead of schlepping all over town looking for a bound copy. Just put your zip code into the locator and then choose the library whose catalogue you'd like to search for availability.
However, if the book you're looking for is out of print or a little more obscure try Alibris.
Alibris is a digital, used bookstore, and one of the best places on the internet to find first editions of your all time favorite or try and recapture those elusive childhood books no one's ever heard of. Though you could always use Alibris for more nefarious purposes and buy a really old copy of some hipster-fabulous book like Howl to start building that coffee house political captial.
Happy reading, kids.