Google Books now offer free downloads of public domain books in an additional format, “EPUB” – a lightweight text-based digital book format that allows the text to automatically conform (or "reflow") to these smaller screens. And because EPUB is a free, open standard supported by a growing ecosystem of digital reading devices, works you download from Google Books as EPUBs won't be tied to or locked into a particular device. Google will also make available these books in PDF format so you can see images of the pages just as they appear in the printed book. To get started, just find any public domain book on Google Books and click on the Download button in the toolbar.
More info: Google Books