I just signed up for the Google Librarian newsletter which was apparently conceived at last year's ALA Conference, so I guess this is perfect timing. Anyway, while going through the simple sign up process and reading through some archival material, I stumbled upon the Superpatron blog who explains how he was able to "insert library holdings information into the Google book search."
Vielmetti writes, "I've taken the code from Jon Udell's Library Lookup and mashed it around until it inserts a link to my library's holdings of that book right on the Google Books screen. The useful bit of data is an ISBN number on some pages that can be turned into the link. If it is too old to have an ISBN, I punted; the better thing to do would be a title search." [there's more here at Edward Vielmetti's blog]
(saw it first from Jodi Healy at Google)