Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
Journal and magazine publishers have used this type of metadata for years. The magazine world uses an extremely large and rich suite of metadata properties and vocabularies collectively known as PRISM (Publishing Requirements for Industry Standard Metadata). There's a new format, just published last year, called the Prism Source Vocabulary (PSV), that is designed to describe the source content for magazines as it is being developed and before it is gathered and published in an issue. The scholarly journal world has long used rich metadata -- typically in what is called the "metadata header" at the top of an XML file -- to make its articles discoverable and linkable. This is done primarily through a service called CrossRef, a nonprofit collaborative service that collects metadata about published articles, issues an identifier called a DOI (Digital Object Identifier) for each of them, and provides the mechanism for a publisher of another article to find the DOIs of all the articles cited in that new article (often scores or hundreds of them). This has revolutionized scholarly publishing: The reader of a scholarly or scientific paper can now just click on the links in the references at the end of an article and immediately be taken to the cited article. (CrossRef even provides a mechanism called CrossMark that enables a user to know whether she has the latest version of a paper.)
Such things are only beginning to be used for books. The reason is that most books are thought of by their publishers, and handled by the supply chain, as discrete "products" that are not online. They are sold online, of course; but what is bought is a distinct product -- print, audiobook, or ebook -- that is consumed offline. They don't connect with each other in any dynamic way.