New Tech Note
Some clarification about checking library holdings from online bookstores:
The check is being done by ISBN, so a bookmarklet can be created for any site (not just Amazon and B&N) that uses the ISBN as part of the URL for the record.
BUT, this means that if the library has the book in a different format (hardcover instead of paperback), and therefore a different ISBN, the record may not show. Of course, you can always click on "Other Editions" and check from those pages.
This probably won't happen very often with science and reference books that are usually only published in one format, but just be aware...