It depends on what formats your device will handle, but there's a wealth of reading material right here on MR, and then there's always the Gutenberg project and ManyBooks.net for freebies.
With an ePub device, you can use the Sony Store or any other online shop that offers ePub format. (Depending on where you're located, there are ways around the geographical restrictions.) Check the Wiki here on MR for lists of where to find free eBooks, eBook stores and eBook libraries.
Many of the eBook stores have programs for publishing through their outlet.
|