what open data services should a public library provide?

Rob Giggey, the lead on Open Data for the City of Ottawa, asks about open data for libraries

quick... if you know good examples of libraries utilizing #opendata (including APIs) please send them my way..c/ @opendataottawa @scilib

— Robert Giggey (@rob_giggey) April 27, 2012

I said I thought there were three main things that a public library can do, plus one done mostly by academic libraries

1. Libraries providing APIs, usually to their catalogue (central book index) – this has been going on for years
2. Libraries providing open data, typically of their catalogue holdings (the database that underlies their catalogue).  Catalogue holdings info is basically what used to show up on the old card catalogue – it’s metadata about the books, authors, but NOT the full text of the book itself.
3. Libraries providing linked data (semantic web), again of their catalogue holdings
4. Libraries providing repositories of full text (typically academic libraries providing open access institutional repositories) with associated APIs

In the context of the Ottawa Public Library (OPL) this would suggest

* Opening of their catalogue layer API to developers (OPL uses BiblioCommons, which has a good API and developer site)

* Their catalogue metadata available through the City of Ottawa Open Data site (as the Brooklyn Public Library's is available through NYC Open Data)

While Linked Data is now widely being shared by national libraries (e.g. US Library of Congress, British Library, French National Library, German National Library, German National Science Library, Spanish National Library) I don't have a good sense of how many public libraries are doing this or to what extent LOD would be used by the public library developer community.

I think there are probably APIs and data for services and content beyond just the catalogue that might be useful to open up.  Beyond that I'm not sure that there are many public libraries using institutional repositories or similar technologies (e.g. OAI-PMH for metadata harvesting).

The Ottawa Public Library has a good handle on consumer-facing technology and interaction including a Library Apps page and an active Twitter feed @opl_bpo

What open data approaches would you recommend for a public library?