WebEnglish.se is in favour of reading real books instead of schoolbooks. There are several marvellous sites that present free e-books to students at all levels.
Epic! for young learners, is organised in categories. Some books have audio and you can even find short educational videos.
Oxford Owl is categorised by age (native), level and book type. All the books have audio and some lack text altogether.
Both Epic! and Oxford Owl are suitable for reading levels from school year 2 to about 7. By 7th grade, the topics may seem childish although the language level is quite high.
Zing! has several categories including the Lexile level of the books. The range is practically limitless. The themes are more educational and less “read for fun”.
For high intermediate and advanced readers, esl-bits has a comprehensive selection of both classic and modern audio-books. As a bonus there are even songs to listen, read and sing-a-long. The domain name registration has expired.