Google Books- Some Titles are not available for download and some are. Ones that are you can use ADE as described earlier. Ones that arent must be read using
Google Apps for mobile devices or you can use your desktop/laptop's (or any javascript supporting) browser
Whether a title is or is not available for download is up to the publisher and in some cases it appears they are testing to see which sells better i.e. some books in a series download-able and others not.
Books On Board sells in a variety of formats
Epub and PDF with ADE DRM as well as MobiPocket, eReader adn Microsoft Reader(.lit)