What we need is even more angels on this pin:
"The Lord of the Rings is not a trilogy: by the time it was being prepared for publication in 1950, Tolkien was thinking of it as a duology: a book of two parts, the other being The Silmarillion - a work concieved of as being of equal size to The Lord of the Rings (Letter 126 to Milton Waldon, 10/3/1950)."
http://www.tolkiensociety.org/tolkien/tale.html