Tolkien conceived of The Lord of the Rings as a single volume comprising six "books" plus extensive appendices. The original publisher split the work into three, publishing the fifth and sixth books with the appendices under the title The Return of the King. Tolkien felt the chosen title revealed too much of the story, and indicated he preferred The War of the Ring as a title.
Tolkien may have hoped to publish the one large volume together with The Silmarillion, and to give names to the individual six books.The proposed title for Book V was The War of the Ring. Book VI was to be The End of the Third Age. These titles were used in the Millenium edition.
The Return of the King was in the end published as the third and final part of The Lord of the Rings, on October 20, 1955.