Openlibrary.org is pretty simple in concept: They want to give every book ever published a web page. Similar in execution to Wikipedia, anyone can contribute to the cause.
Say that you find your favorite book on the site, but the one that’s been uploaded is not in perfect condition, you can then grab and scan your own pristine copy to further improve the cumulative efforts.
This is one of our favorite sites on the web for obvious reasons. We were able to find books on everything from the physics of flying to the beginnings of astronomy. Open your mind on your favorite subjects by visiting the link here.