PDF is by far the easiest to acquire; .epub is the most common & best-featured ebook format for portable readers. The Sony Reader will read any PDF–but most of them are formatted in a way that makes them hard to read; using the “M” and “L” options loses the formatting, but only some books are easy to read that way. (It’s fine for novels; bad for charts & tables & anything with pictures.)

You can get free ebooks from and, along with several other sites; feedbooks even has PDFs size-formatted for reading on the Sony Reader. And Mobileread.com has thousands of free ebooks, plus forums and a wiki that can help you figure out what’s available and whether stuff the Reader can’t read can be converted.