A "book trilogy" refers to a series of three books that are related to each other, often telling a continuous story or exploring the same theme. Full definition
The films are adapted from the popular young - adult book trilogy of the same name, written by James Dashner. (weliveentertainment.com)
Snoopy from «Peanuts» comic strip by Charles Schulz; Shiloh, hero of children's fiction book trilogy by Phyllis Reynolds Naylor. (soonervet.net)
The Lord of the Rings book trilogy represents, by any measure, some of the most popular books in the history of English literature. (relevantmagazine.com)