Harry, I wondered which public libraries as well, but, being a newbie, thought everyone would already know!
Since I started this thread I've been exploring further options and have found that Books on Board is one of the better sites - I download the Mobi version and crunch it through Calibre (which is brilliant!). The new PRS-505 upgrade for Adobe is useful too, but I'm not fully happy with having to use the Sony book library again and haven't yet been able to work out how to add downloads to Calibre. This means I've got two libraries on the go, which is not ideal!
Incidentally, I popped into my local Sony shop the other day and, amongst other things, asked them about when their Readers will be on sale (Sept 14th). The staff had a sample under the counter, still in its sealed box, but they barely knew what it was for! Let's hope they have a little training before they try and start selling the thing!!!