I hope its OK to post here for UK readers/users?
UK libraries are slowly coming on-line (at last), and here are a list of the current collections and addresses for those that I know of.
I've only listed ebooks, and not the audio-books.
Manchester UK
http://manchesterdownload.lib.overdrive.com/
Ebooks
Fiction 967
Non-fiction 600
London libraries consortium
http://llc.lib.overdrive.com/
Ebooks
Fiction 926
Non-fiction 503
Essex
http://essex.bookaisle.com/
Ebooks
Fiction 485
Non-fiction 342
Liverpool
http://liverpool.lib.overdrive.com/
Ebooks
Fiction 623
Non-fiction 209
York
http://yorklibraries.lib.overdrive.com/
Ebooks
Fiction 138
Non-fiction 42
Surrey
http://surreycc.lib.overdrive.com
Ebooks
Fiction 264
Non-fiction 185
I can't speak for Scotland Wales or Northern Ireland, but here are a few of the ones I'm using and a list of their joining requirements;
Liverpool - allow you to join on-line. States that they will send membership card to you in the post, but you then have to call into any Liverpool branch to have card authorised.
London - allow you to join on-line, but you have to collect your library card from a London library. Proof of UK ID required at time of Pick-up.
Manchester - allow you to join on-line but you have to collect your library card from a Manchester library. Utility letter required to prove your address (anywhere in the UK allowed)
Surrey - allow you to join on-line as an elibrary member, will email you a virtual library card. Anyone in the UK can join.
Essex - allow you to join on-line. Emails you a membership number immediately upon application. Anyone in the UK can join.