I love reciprocal borrowing. One strange thing I found when I moved out to Washington was that all the libraries weren't interconnected. You could only borrow books in the town that you lived in. This meant a drastically reduced collection.
Until now. Recently our branch has begun a pilot program with several local libraries. If you have a positive borrowing record, you may apply for a library card at any one of the those libraries (or all three, in my case). Though you do have to return the books at their respective libraries, the huge collections make up for it.
I'm off for a little shelf browsing!