An amazing book, one that my mom picked up at a bookfair in Oklahoma City many years ago, and i read it in high school. It should be included among the classic sci-fi/fantasy books for grades 4-6, but unfortunately, it's out of print, and although I reccomend it to everyone, I don't think it will ever gain the popularity it deserves until someone repreints it. Meanwhile, it can be gotten easily from Amazon or borders :)
As for a plot, Christopher, on a dare, goes into this deserted house and finds a man living there who has been waiting for him, apparently. He is a teacher, and, through Merlin-like abilities, teachers Christopher about himself and about life in general.
This is one of those books that, if my house ever caught on fire, and I had time to, I would grab it (and my signed copy of Eragon). I know right where it is.