If you and your students are beginning to explore e-readers, such as the Nook, Sony Reader or Kindle, you might want to explore the free ebooks available at the Internet Archive. Recently, the books are being made available in epub and other formats suitable for portable ereaders that make them easier to use on a range of devices.
The site includes public domain books scanned from libraries around the world and integrates texts from other collections such as Project Gutenberg and span most every topic imaginable.
One example I ran into in my own research is “Radio in Wartime“, a 1942 book by University of Chicago professor Sherman Dryer and published in 1942. It looks at the many political issues surrounding the radio broadcast industry at the time and the uneasy relationship between the radio networks and Federal government, as well as studies on effective uses of broadcasting for public information and education.
The book is available in epub, pdf, Kindle and other formats. The epub version has been scanned using optical character recognition so it is properly formatted for ereaders – since the conversion process is automatic, there are some odd page breaks or other anomalies, but the book is very useable in it’s ereader form.