The OCLC website is now carrying details of the billionth holding in WorldCat. It was entered by Worthington Libraries for The Monkees: the day-by-day story of the '60s TV pop sensation
There are more details on the WorldCat page, including a list of libraries who contributed holdings on either side of the billionth one!
This is a major milestone based on the holdings of thousands of libraries worldwide! It is the richest source of book data that we have, as well as a major reservoir of data about music, maps, sound and video recordings.
An important note: a holding is not an individual copy. If a library has one hundred copies of the latest Harry Potter this counts as one holding.