Why is the book called The Lord of Rings when plot only about one ring?

The title "The Lord of the Rings" refers not to a single ring, but rather to the Ruling Ring, which is the most powerful of the magical rings created by the Dark Lord Sauron. Throughout the story, the Ruling Ring is the central object of desire and the source of much of the conflict and tension. It is the Ring that Sauron seeks to regain in order to restore his power and dominate Middle-earth. The title thus highlights the importance and significance of the Ruling Ring in the overall narrative of the book.