Lord of the Rings is also a series of books, if I recall correctly (I don't know whether J.R.R.T. intended this, or if it was a publishing "artifact").
It was written and intended to be read as a single story. The publishers thought that it would never sell, so they insisted that it be "modularized" into three "books", with separate titles.
Later, of course, it was published as a single volume, as Tolkien intended all along.
Andrew