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I just discovered that the Library of Congress has established a persistent URL (or "permalink") for every item in their catalog. The data in the link is taken from the item's Library of Congress Control Number (LCCN) and is therefore called an LCCN permalink.
But the important thing is that we have a unique, machine-readable, identifying "name" for practically every book in the universe, established by a respected authority. The semantic web (or "Web 3.0" as it's often called now, although it predates Web 2.0 by several years) is chiefly concerned with expressing and collecting information from various sources automatically.
If web applications use this identifier to refer to a book, then one can search the semantic web by the identifier to find out about the book. For instance, here is the LCCN permalink of a book I bought yesterday. Sites that sell books can indicate if they have the book in stock. Library sites can indicate if they have it for borrowing. Reading courses can name it in a list of titles. Social networking sites can indicate who's listed this title as one of their favorites. And because of the uniqueness of the identifier, the implications that can be drawn from it are unambiguous.
Library scientists are very interested in managing giant systems of information (or metadata), so I'm glad that this particular library is helping enrich the semantic web.