This has been documented in the Lord of the Rings movies' extended edition bonus materials (for The Two Towers if I recall right). Tolkien originally wrote Lord of the Rings as a single volume; at the time, paper was expensive, and publishing as a single volume would have been expansive, and it would have to highly priced, limiting the potential readership. The publisher, against Tolkien's wishes, decided to split the book into three volumes. Which led into other issues such as how to title each volume.

This has been documented in the Lord of the Rings movies' extended edition bonus materials (for The Two Towers if I recall right). Tolkien originally wrote Lord of the Rings as a single volume; at the time, paper was expensive, and publishing as a single volume would have been expensive, and it would have had to be highly priced, limiting the potential readership. The publisher, against Tolkien's wishes, decided to split the book into three volumes. Which led into other issues such as how to title each volume.