DOI is short for Digital Object Identifier. You can look for the DOI as part of a book’s bibliographic information. Not all books have DOIs in the BHL collection. To date we have assigned DOIs to books, but only to a small selection of journals. We are considering assigning DOIs to articles for the future. When citing a work using its DOI you are referring to a permanent fixed reference that can survive beyond the life of its URL for example. To learn more about DOIs, please see https://en.wikipedia.org/wiki/Digital_object_identifier.