I emailed Waterstones about their ebook list and got this response:
Thanks for your enquiry to Ask a bookseller. At this point in time we are not in the position to give a full list of eBooks that will be available to download on our site. The eBooks themselves won't actually be visible on Waterstones.com until early September, but the banners currently on our site and the posters in some of our stores will give an indication of what types of books to expect.
I have also listed below the 100 Classic eBooks that will on the free CD that accompanies the Reader from Sony. We are working very closely with our publishing colleagues to ensure an exciting selection of eBooks will be available to our customers for the launch of the Reader from Sony and well beyond
It seems to me that the only reason for withholding the list is because it isn't much of a list - otherwise they'd be ranting about it.
Holding off on the pre-order til I know more.
And Gottaa, the reader does support more formats now, this is from the tech specs on the Sony site:
Compatible Formats
Unsecured / free eBooks & text files:
EPUB eBook, BBeB Book, Adobe� PDF, Microsoft� Word, TXT, RTF
DRM (Secure / purchased) eBooks and text files:
EPUB eBooks (Adept) BBeB eBooks (Marlin)
Unsecured Audio files:
mp3, AAC (not most DRM audio)
Image & Photo files:
JPEG, GIF, PNG, BMP