The Lord of the Rings was first published in three volumes. The volumes were The Fellowship of the Ring, The Two Towers, and The Return of the King, released between 1954 and 1955.
Tolkien had conceived the work as one large continuous story rather than three separate novels. Its division into volumes was primarily a publishing decision: the book was expensive to typeset and print, and splitting it reduced the financial risk for Allen & Unwin. Each volume contained two of the work’s six internal books, along with additional front matter or appendices.
This publishing format is why The Lord of the Rings is often called a trilogy, although Tolkien sometimes objected to that description. He regarded it as a single work, not three independent novels. Later editions were also printed in one-volume form, including a single-volume edition published in 1968, but the original publication was the familiar three-volume set.