FBReader will read content in these formats: FictionBook fb2, Plucker pdb, aportis doc, Weasel zTxt, html and text.
As far as I know, no online bookstores sell any e-books in these formats. Well, take that back -- some smaller publishers have long sold their titles in html. As does Baen, both with its websubscriptions and its free e-book library.
FBReader does not read any e-books in proprietary, DRMed formats, which is what the e-bookstores deal with. No e-reading programs that do handle such formats run on the Nokia 770 at present.