What's interesting is that not all "full view" books are available for download. I wish there were a filter for just downloadable books.
Are most of the examples you have seen been books published recently enough to still be copyrighted? That has been my experience, in the subjects I have searched. A sufficiently old book (one definitely in the public domain) seems to have had a .PDF available for full-text download for a while, and now the new .EPUB format too.