We Give Books is a free site with free children's books and lesson plans. So far the books are available for kids up to age 10. Once you have registered, (free) you can access any of the available books on your e-device. The site has links to individual book lesson plans as well as literacy skill specific ideas, for grades k-12.
The site, I believe is a non-profit site. For each book read online, real books are donated to charities for kids to enjoy in person.