American English E-books
http://www.americanenglish.state.gov/ebooks
Read e-books anytime, anywhere: on the go or in the classroom! Free American English e-books-including classic graded readers, an idiom reference guide, and resources for teachers-are available in .epub (most e-readers and tablets) and .mobi (Kindle-compatible) format in the “Downloads” section of select American English resources.
Open Culture E-books
http://www.openculture.com/free_ebooks
A resource for finding freely available classics in e-book form, including fables, fairy tales and books in the public domain. Some titles can be read online, other can be downloaded to e-book readers.