What’s a DOI? Here’s a short description from CrossRef
The DOI, or digital object identifier, serves as a persistent, actionable identifier for intellectual property online. DOIs can be assigned at any level of granularity, and therefore provide publishers with an extensible platform for a variety of applications. And DOI links don’t break. Even if a publisher needs to migrate publications from one system to another, or if the content moves from one publisher to another, the DOI never changes.
While the use of DOIs for book chapters is especially interesting, DOIs can be utilized for smaller book sections as this example of an entry for Ian Fleming in the Oxford Dictionary of National Biography illustrates. (Notice the DOI, "Ian Lancaster Fleming (1908â€“1964): doi:10.1093/ref:odnb/33168," at the bottom of the entry.)