Also, don't forget all the free books on this site. I find that some of the free EPUB format books from Google and Kobo are a bit of a mess as the scanning is not 100% foolproof. The free books on this site, Feedbooks and http://manybooks.net/
have been "proofread" and contain less errors. Also I have to recommend http://ebooks.adelaide.edu.au/
from my own Adelaide university - they are all formatted very well and a real find for anyone studying as they have a lot of standard texts.