I'm not sure of how the DRM works on sites like Amazon and Barnes and Noble, but at least for the former I believe you can set it up to have book delivered in epub format to the device of your choice. I'm not sure though.
Libraries often have books that you can borrow in digital format for free. Again, these have DRM installed which can make it hard to use them flexibly, but they usually come in many formats for use on almost any device.
If you find a good source of free, public domain ebooks in epub format let me know. I would love to make them available on this website--without any DRM. Of course, it is incredibly important that in terms of copyright they are in the public domain.
Online Book Club
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"Only the descent into the hell of self-knowledge can pave the way to godliness." -Immanuel Kant