I’ve taken more from Wikipedia than I normally do because it’s useful to illustrate the point:
The Lord of the Rings is an epic fantasy novel written by philologist and University of Oxford professor J. R. R. Tolkien. The story began as a sequel to Tolkien’s earlier, less complex children’s fantasy novel The Hobbit (1937), but eventually developed into a much larger work. It was written in stages between 1937 and 1949, much of it during the Second World War.Although known to most readers as a trilogy, the work was initially intended by Tolkien to be one volume of a two-volume set, with the other being The Silmarillion. However, when Tolkien submitted the first volume entitled The Lord of the Rings to his publisher, it was decided for economic reasons to publish the work as three separate volumes, each consisting of two books, over the course of a year in 1954–55, creating the now familiar Lord of the Rings trilogy.
Anyway…. The novel is split into The Fellowship of the Ring (sections 1-22), The Two Towers (sections – 23 – 43), and The Return of the King (sections 44-62). You can, of course see a larger version by clicking on the image, but it should be pretty obvious that the book divisions are fairly arbitrary.