The first three books of "The Wrinkle in Time" series. The first three books of the Earthsea cycle by Ursula K. LeGuin. These both skew young but I valued re-reading them as I got older: The Dot and the Line: http://en.wikipedia.org/wiki/The_Dot_and_the_Line The Phantom Tollbooth: http://en.wikipedia.org/wiki/The_Phantom_Tollbooth
Why does everyone keep recommending books for teenagers that I read in elementary school?
I mean, hell, if we're going for epic high fantasy set in the (recognizably) modern world, try So You Want to be a Wizard by Diane Duane.
And to those of you who tried taking the Oath when you were a child, yes, the rest of us did, too. Pity it didn't take.