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 that has no published, official title is less clear. For example, would it be the Harry Potter series (referring to the titular character) or the Harry Potter series (referring to those seven novels by J. K. Rowling)?
It makes sense to italicize Harry Potter, since the name appears in all the titles of the books. Another example would be Isaac Asimov’s Foundation series (which for a long time was the Foundation trilogy), since all its books contain the word Foundation.
Published 12 September 2017