I discovered something called WOWIO when I learned today that K. Eric Drexler had published an updated 2006 version of his 1986 nanotechnology classic Engines of Creation. It's available as a free ebook, but only at WOWIO. WOWIO?
WOWIO is web site that offers free downloadable e-books (PDF only) to registered users (currently only U.S. users). Registration is the key - they ask you questions to be certain you are a real person, and to get some idea of your interests and tastes (yup, marketing). They say they share this information with their publisher sponsors "only in aggregate," and it is this sponsorship that allows them to offer the books for free.
Some of these works are long out of copyright and available other places for free (e.g., Frankenstein, The Art of War), but others are quite recent and interesting (to me anyway!). For example, in addition to Engines of Creation 2.0, I downloaded The Ascent of Science by Brian Silver, a history of science that I own in paperback. I've been reading sections of it but it's too big to carry on trips. Now I have an electronic copy on my notebook. I also downloaded Stephen Hawking's The Theory of Everything and Kurt Vonnegut's The Sirens of Titan. They even have comics and graphic novels.
I don't really need more books, but this is mainly about portability. We'll see how it works and if I actually read ebooks on my screen. I have had various ebooks on my PC for years and frankly have read very few of them, usually only short SF stories.