Joel Osteen's book has just been released on paperback. I'm going to deal with it like I did the popularity (among aging liberal 60-somethings who were raised in fundamentalist churches where they were told that God hates short pants) of John Shelby Spong's Jesus for the Non-Religious: displayed books on Christianity all around it. Heh.
While we're on the topic, Christianity Today had a worthwhile article on the prosperity gospel in Africa a couple of months back, "Gospel Riches."
Joel Osteen is not a Christian teacher. He might be a Christian man, but he does not teach Christianity. If you'd like a brief explanation and critique of the prosperity gospel, I direct you to this post by the Internet Monk, as well as his criticism of Joel Osteen himself.