These are unauthorized by J. K. Rowling, but if you know a good bit of mythology or ancient-or-just-old theology, they make some good points against that theory:
Colbert, David. The Magical Worlds of Harry Potter: A Treasury of Myths, Legends, and Fascinating Facts. Wrightsville Beach, NC: Lumina Press, 2001.
Kronzek, Allan Zola and Elizabeth Kronzek. The Sorcerer's Companion: A Guide to the Magical World of Harry Potter. New York: Broadway Books, 2001.
Colbert's pretty off on a few things--the Kronzeks not so much--but they get the point across. In a nutshell, the way-above-third-grade stuff's in there, if you know the background and it actually rings a bell when you come across it. Rowling put much more work into the world of Harry Potter than the actual plot. The books also abound with symbolism, and when one reads the first four in close succession, the tiny details that seem insignificant in the first or second books pop out later on as distinct and sometimes damning clues.
The question now is whether she can keep it up for the next three books. ;)