I don't think there are any sites that have ALL of their books for free, but one app I have recently discovered connects you to your library and allows you to check out two different formats of eBooks and one audio book. I'm not sure that this is exactly what you were looking for, but the app is called Overdrive and is available for iPhones. I can't go into great detail, but it really is a useful app if you're interested in free eBooks. I hope this helps!
I know you said books, but I thought I'd mention this anyway. There is a site called mangafox where you can read thousands of japanese comics(manga) for free online. You can browse by genre and search completed and ongoing series. All titles are translated to english. Just search " free manga online" or something similar for more sites. It might be a little weird at first if you've never read manga, but there is a lot to read if you're bored.