Hello! I am also a new poster to the forums!
I have had great success buying Microsoft LIT books from Fictionwise. I use ConvertLIT to remove the DRM (although you will need to use the executable <clit> from the command line so as not to expand the original lit file too far) and Calibre to convert the new deDRM lit file to the lrf format needed for the Sony. I then import the Calibre lrf file into ebook library and finally load it onto my new Sony PRS-700. It takes a little work, but the results are worth it! (There might be a shorter process, but this one works for me. )
You can join the Fictionwise Buying Club which helps a little with the price and you earn Micropay cash which can get you free books. You can also get free classic books from Project Gutenberg and convert those to the Sony format. I have had an ebookwise 1150 for a couple of years and absolutely love it, then my wonderful family gave me the new Sony for Christmas! I am a total book junkie and a techno-geek and the electronic books have made it possible to combine the two!
I have been using MobileRead for quite a while to help debug problems I encounter along the way but it took getting my second reader to finally join the forums. Thanks to all for the help over the years!