What are all the books in the Lord of the Rings trilogy?

Context: Although often called a trilogy, the work was intended by Tolkien to be one volume of a two-volume set along with The Silmarillion. For economic reasons, The Lord of the Rings was published over the course of a year from 29 July 1954 to 20 October 1955 in three volumes titled The Fellowship of the Ring, The Two Towers, and The Return of the King. The work is divided internally into six books, two per volume, with several appendices of background material. Some later editions print the entire work in a single volume, following the author's original intent.

Answer: The Lord of the Rings trilogy is comprised of: The Fellowship of the Ring, The Two Towers and The Return of the King.