An ISBN, or International Standard Book Number, is a unique numeric identifier that is used by publishers, libraries, booksellers, and other retailers in the marketing and buying of books. ISBNs are not included in works-cited-list entries for a number of reasons. First, ISBNs were developed only in the second half of the twentieth century, so older books won’t have them. Second, an ISBN doesn’t help readers directly access the book’s content. An ISBN does contain information, however, about a book’s edition. When a publisher releases a new edition of a book, a new ISBN is assigned. The edition you cite is important, so be sure to include in your works-cited-list entry the edition number for an edition that is not the first:

Fairer, David, and Christine Gerrard, editors. Eighteenth-Century Poetry: An Annotated Anthology. 2nd ed., Blackwell, 2004.