EPUB files can be DRM'd (the ePub format does not specify a method of DRM, so the method and device support can vary from file to file). 'Secure' is just a euphemism for DRM in this context. Avoid them like the plague.

I'm also a fan of http://manybooks.net

I just started reading the book 'Makers' by Cory Doctorow, who publishes all of his books under Creative Commons licenses and provides free downloads on his website: http://craphound.com/?cat=5 Really digging it so far, and it's free (as in beer *AND* speech)

If you're at all interested in the legal/political climate (in the US) that prevents you from being able to legitimately purchase reading material for your devices I highly recommend the book 'Free Culture' by Lawrence Lessig It's a short, informative and enjoyable (though at times infuriating) read.