The Lord of the Rings developed as a personal exploration by Tolkien of his interests in philology, religion (particularly Roman Catholicism[12]), fairy tales, Norse and general Germanic mythology,[13][14] and also Celtic[15] and Finnish mythology.[16] Tolkien acknowledged, and external critics have verified the influences of William Morris[17] and the Anglo-Saxon poem Beowulf.[18]
Added note to the other responses: The Lord of the Rings was written as a single novel, not as a series. Only the publisher, upon accepting Tolkein's massive work, split it into the three present volumes.
From Wikipedia: Although generally known to readers as a trilogy, Tolkien initially intended it as one volume of a two volume set, with the other volume to be The Silmarillion; however, the publisher was not interested in the second volume and in 1954-5 printed The Lord of the Rings as three books rather than one, for economic reasons.