there are about 3000 hand made ebooks downloadable for the Sony at
http/www.mobileread.com .
Other books for the Jinke/Cybook/Bebook and Iliad and others, you can download there too.
You can even make your own ebooks there, and upload them if they are copyright expired.
Find better versions then on project gutenberg (often layout optimized for these readers).
I would only recommend you a Sony reader (I own the PRS 505 and it is one of my most fun gadgets around!) if you are an avid reader, or just love to code books from HTML text to the appropriate format.
The device is slow, but the good thing about it is, that it uses e-ink technology;which almost looks like a magical text appears on a white plastic background.
It looks much better then reading books on your laptop.
Apart from text files, the device also has the capability to view greyscaled pictures, pdf, and playback mp3 files (which makes little sense to do,unless you're listening to an audiobook on a low bitrate compression).