Books
Feedbooks - many books are in public domain, some need to be purchased.
CK-12 - Flexible textbooks
Booklamp - Was described as the "Pandora" for books. You put in the book title you like, the site will find other books that are similar to that book.
Storyjumper - Book authoring site. Can do picture books. However, must use available art. Cannot upload your own pictures.
Project Gutenberg - Most books that are available in the public domain.
LibriVox - Provides free audiobooks from the public domain.