Free books that are available online, that is. The grand daddy of free literature is Project Gutenberg. A similar site is Manybooks. An amazing commercial site is the Baen Free Library. This is a library of free books by current science fiction authors that may be downloaded. Lastly, if you want something to listen to in the car or while mowing the lawn, Librivox has you covered.
All of the books from these sites are available to be downloaded in a plethora of formats. All are free from DRM nonsense. All but the books from Baen are in the public domain. The Baen books are still (mostly) covered by the authors' copyrights, but you have their permission to download, print, copy and read.