J.R.R. Tolkien divided The Lord of the Rings into six internal books.
Although the work is commonly sold and discussed as a trilogy, Tolkien conceived it as one long novel organized into six books, together with appendices. Publishing constraints led George Allen & Unwin to issue it in three physical volumes: The Fellowship of the Ring, The Two Towers, and The Return of the King.
The first volume contains Books I and II, the second contains Books III and IV, and the third contains Books V and VI. This structure explains why the volume divisions and the internal narrative divisions do not match exactly.
The phrase “trilogy” is therefore useful for describing the publication format, but “six books” is the precise answer for Tolkien’s internal organization. The distinction is also why editions and scholarly discussions may refer to both three parts and six books without contradiction.