In July 2010, The Chief Officers of State Library Agencies (COSLA) released a report [pdf] on recommendations for ebooks in public libraries. The COSLA report endorsed the exploration of BookServer OPDS Catalogs to enhance the discoverability of ebooks held by libraries. The report suggests that coordinated aggregation by search engines would facilitate library patrons finding electronic books to borrow and read more easily.
With the permission of COSLA, I have excerpted the use case ("Scenario Two") that was included in the full report, as an excellent and simple introduction to the promise of OPDS Catalogs in libraries.