Hi Justin - I'm part of the team at NYPL Labs who'll be helping out at the event.
There are a couple of sources of library data that'll be available.
1. Our catalog API is at http://developer.bibliocommons.com/. This is the API in front of our new catalog interface http://nypl.bibliocommons.com. It's usually only for library partners only, but Bibliocommons is opening it up for developers at the event. You can sign up now, but to be approved, you'll have to mention "hackathon" in the registration. It's also got a limit of 5,000 calls per key per day, so it's not ideal for scraping, but it's good for searching, structured records, structured lists, and making lending possible.
3. For any big data stuff, the best bet is the OpenLibrary data feed or API. http://openlibrary.org/developers It's an amalgamation of book data from the Library of Congress and a bunch of other libraries. Couple million book records.
If there's anything else you're interested in, let me know and I'll see what I can open up. I'll be there all weekend so I'm looking forward to meeting you.