My wife works for the Horsham / Hatboro library. I built her a bookmarklet.
Here is how it works: Go to amazon, search or browse for a book, and click on the bookmarklet. You should see the listings for this book on Montgomery County, PA libraries.
If you get a session expired message from mclinc, try again. No titles means the book doesn’t exist in the library system under that ISBN. (The bookmarklet passes this to the library search.) Try a different printing or edition.
The whole thing is based on Jon Udell’s LibraryLookup Generator.