Another place for technical epub books is InformIT. DRM-free, but they add your name to the book title "Blah blah (Your Name's Library)".
They also do good deals of the day which are a significant discount.
http://www.informit.com/index.aspx
For more general books I noticed that Foyles in the UK sell Adobe DRM-ed pubs. They might be worth a look if Waterstones etc don't have what you're looking for.
http://www.foyles.co.uk/