When a trilogy is published in one volume with a title of its own, the course of action is clear: italicize the title of the trilogy as if it were a work. Naguib Mahfouz’s The Cairo Trilogy, containing the novels Palace Walk, Palace of Desire, and Sugar Street, is an example. So are Ursula K. Le Guin’s The Earthsea Trilogy and Pat Barker’s The Regeneration Trilogy. Margaret Atwood’s The MaddAddam Trilogy is another but different example: its three novels—Oryx and Crake, The Year of the Flood, and MaddAddam—are published not in one volume but as a boxed set (Anchor, 2014).

The question of how to style a trilogy or series of books or movies that has no official title is less clear. For example, would it be the Star Wars movies or the Star Wars movies? Your decision will depend on what makes sense for the particular body of work. Star Wars is the name of the first movie released in the series. Since the title is foundational, italicize the series name: Star Wars movies. If you are writing about the Nancy Drew books, style the series name roman, since “Nancy Drew” does not appear in the titles of the individual books. If you are discussing the Harry Potter books, you could style the series name either way—Harry Potter books or Harry Potter books—since the series is associated with the first title in the series (Harry Potter and the Sorcerer’s Stone) and also with the character’s name.