Baen Books Science Fiction & Fantasy - another eBook dealer that doesn't have much DRM and offers books, for example, as HTML. Fictionwise has a wider selection, but Baen is specialized in science fiction and fantasy.
Archive 14. March 2009
Calibre - free software for managing eBooks and synchronizing with the PRS-505. Available for Windows, Mac, and Linux. Update: this thing is great! By far better than the included Sony software - and together with Adobe Digital Editions, you can easily buy your books from Thalia and transfer them to the device without any problems. Due to the DRM on books from regular sources, you need to use Adobe Digital Editions for reading (and probably for downloading as well), but otherwise you can fully use Calibre as a replacement for the somewhat cumbersome Sony software.
Sony Reader PRS-505 - LatheWiki - tons of links about the PRS-505, including how to patch the firmware yourself and links to programs with which you can manage the PRS-505 on systems other than Windows.