I thought this service might be useful to you. I think it's great. It basically keeps track of all your library books (from different libraries) which are out, and sends you email reminders "before" they are due. It works with RSS readers as well.
Take a look at their demo (http://www.libraryelf.com/Demo.aspx).
It only takes a few minutes to signup and add your library cards to a single account.
Give it a try if interested -- http://www.libraryelf.com