The store offers over 100 thousand titles with prices ranging from £1 to £13. You can also find periodicals there. Newspapers including The Guardian, The Daily Mail, and The Observer can be bought on the site by the issue. You can also get the newspapers via a monthly subscription.
The ebooks can be read via the Sony Reader Wifi as well as most ereaders on the market (everything but the Kindle, basically). Sony also offers reading apps for Windows, OSX, and Android, but I would recommend that you avoid the OSX and Windows apps; they are horrible.
I have a good reason to heckle Sony by calling them the smallest major ebookstore; I don't think the UK ebookstore should have been opened. For the longest time I have beveled that Sony would be better off partnering with someone else's ebookstore. Let them do the heavy lifting and shoulder the development cost. This would save Sony money and offer a better experience to customers, and that's exactly what Sony did when they launched down under back in 2010.
But like some readers have commented, Sony doesn't know how to give up on unprofitable product lines. And that's going to be their undoing.