Voluminous finds free books on the Internet. It makes a catalogue of every book it finds, and you can search this, and download any book you like. You can read the books from within Voluminous, or export them to read elsewhere.
Now, since the Mac has PDF creation built in, it wouldn’t be too much of a stretch — perhaps with some AppleScripting — to take plain text files and automagically make them Sony Reader-optimized PDF files.
Hmmm … maybe this developer can even include such a script with the program?