I haven't read the book, so I have no opinion yet. But because of the controversy, because we are doing Desiring God in our men's small group, and because Piper has been a big part of my formation of a theology of suffering (through his messages on Katrina and previous natural disasters), I provide this link for those who might be interested.
Read with discernment. This is true of all books written by man. There is only one Gospel, and I love the way Paul puts it in Galatians 1:6-9:
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
Paul was so sure of the Gospel he had preached that he wanted to make it clear that there was no other, and he uses the strongest language against those who would teach any other. Any man (even John Piper) runs the risk of getting it wrong--we must be Bereans and always examine every work of man through the lense of Scripture.