The Seattle Public Library is offering RSS feeds that will alert users to overdue books and notify them when a new book is available by their favorite author according to NPR's Future Tense, Libraries Get Hip to RSS.
Sirsi is pointed to as the first library automation vendors to offer RSS feeds with their catalog software. Any search that can be run in the Sirsi catalog can be saved as an RSS feed.
Also quoted is Steven Cohen, of Library Stuff, who points out that while there is limited demand for RSS feeds right now, it will likely increase significantly in the near future. How much better to be in front of the curve rather than behind it!
And once RSS is more commonplace, both public and private libraries could deliver new book lists on specific topics as customized RSS feeds. Let's just hope the REST of the library automation vendors catch up with Sirsi.