In a title like Gardner’s Art through the Ages, should Gardner be listed as an author or a contributor?

To determine what to list in the Author or Contributor element of a works-cited-list entry for a book, consult the book’s title page.

The title page for the fifteenth edition of Gardner’s Art through the Ages lists Fred S. Kleiner as the author. The original author of the book, Helen Gardner, is not listed, so you would include her name only in the work’s title: 

Kleiner, Fred S. Gardner’s Art through the Ages: A Global History. 15th ed., Cengage Learning, 2016.

If the work has an editor instead of an author, list the editor’s name in the Author element, followed by a label:

Fulk, R. D., et al., editors. Klaeber’s Beowulf. 4th ed., U of Toronto P, 2008.

Note, however, that if the work is an edited collection of material by one author, the author’s name should be listed in the Author element:

Milton, John. The Riverside Milton. Edited by Roy Flanagan, Houghton Mifflin, 1998.