In Young's story, the shack is a metaphor and it represents a place in our lives where we get hurt and then emotionally remain there in our pain, insecurities, & doubts. In God's grace, He brings us back through our shack in order to help us deal with our pain. Ultimately, it's a story of redemption, healing, & hope. In the story God meets Mack (the main character) right where he is in his hurt & insecurities. Then God marvelously reveals himself to Mack and heals him through his unconditional love. It's a great story!
But you have to remember, it's a story! It's a metaphor! It is not be read as systematic theology. This is where the controversy comes in. Young has a very interesting way of disclosing the Trinity in his story. But, it is not heretical! Young addresses the very difficult doctrine of the Trinity with orthodoxy and creativity. In no way is his depiction and/or description of the Trinity heretical.(Granted, pockets of it do seem weird and may make people of uncomfortable, but collectively it is very sound.) In writing story (which, by the way, was written for his children) Young wanted to depict a very vibrant, robust, and experiential view of the Trinity. He had this Ravi Zacharias quote about the Trinity in view as he was writing - "unity in the diversity in the community of the Trinity." That's a beautiful and accurate description of the Trinity. So don't get your undies in a bundle!
Here are some reviews on The Shack:
Here's some thoughts from Mark Driscoll on The Shack. (Mark doesn't have the nicest things to say about it! He gets pretty fired up and I agree with Mark's view of the Trinity. I think he misreads Young though.)
Here's a nice clip from the 700 Club (of all places!!!) on Paul Williams and The Shack:
Pick one up for yourself and see what you think of the book: