A Digital Object Identifier (DOI) is a unique and persistent identifier which provides a link to an object on the Internet via a registration and indexing service.
DOIs are often used in citations and discovery services to provide permanent links to online articles, ebooks, images, reports, and other types of digital objects. The DOI acts as a pointer to the actual location of the item.
Publishers now regularly assign a DOI to each journal article or ebook (or ebook chapter) they publish. If the article or ebook moves to a new location, the publisher updates the DOI index to point to the new location. The DOI URL never changes.
Many times, when a link fails, an item still can be located using its published DOI. Look in the record for a DOI that may lead to the correct location. See the DOI Tools tab for browser extensions that can help with this.