University of Virginia eBook library; http://etext.lib.virginia.edu/ebooks/ (free)

Project Gutenberg www.gutenberg.org/ (free)

Bookshare.org www.bookshare.org/ (some free)

Amazon; www.amazon.co.uk (cost)

Stories from the Web www.storiesfromtheweb.org/

MAPE www.mape.org.uk/activities/bigbooks/
http://www.newi.ac.uk/englishresources/

http://www.firstschoolyears.com/

BBC, BECTa, NGfL, LTS ….