Google Books. This site has full versions in .pdf format of a lot of out-of-copyright books; it's a great resource. To find the full version, use Advanced Search. Note: for-profit print-on-demand publishers have locked up some of these these out-of-copyright books so that Google can't provide a full view. If you can't find them at Google Books, try archive.org or Project Gutenberg.
Archive.org. This site has books as well as sound and film media, including some that are not available at Google Books.
Although its search function can be frustrating sometimes, Archive.org is an excellent source for books, especially those that have been locked up in Google. Also, if you have a good internet connection, the "flip book" function is a great way to read books and a nice break from reading in .pdf format. See, for example, this version of Crane's The Black Riders: