Which Tolkien novel is divided into six books but usually published in three volumes?

The Lord of the Rings is divided into six books but is usually published in three volumes.

Tolkien wrote the work as a single long novel, not as a trilogy of separately conceived stories. Its six internal books were published in three volumes: The Fellowship of the Ring in 1954, The Two Towers in 1954, and The Return of the King in 1955.

The three-volume format reflected practical publishing considerations, including the size and cost of the manuscript in the postwar period. The title of each volume therefore does not correspond to one internal book; each volume contains two books, along with appendices in the final volume.

Calling it a trilogy is common and understandable, but “six-book novel published in three volumes” is more precise. The structure also helps explain why some editions and adaptations organize the story differently from Tolkien's original internal divisions.