Anything that is in the public domain is free to download. Every year many new books become part of the public domain as their copyright expires. Many PD books are available free in the iTunes bookstore, and I also highly recommend the app Stanza, which allows you to search many online sources for books, including Project Gutenberg which is an open source PD repository. Google Books app is also good for searching books that are in the PD or that Google has scanned.
Of course PD works can also be marketed by publishers, so make sure you don't pay for anything that should be free! For example You can download Moby Dick for free or for money in ibooks bookstore, so best to search Project Gutenberg first.
Hope this helps!