There doesn't seem to be much Catholic comment on this book yet (I'd be grateful for any links) but there is a helpful review by Julie D. at Catholic Media Reviews. Here is her brief summary of the plot:
Essentially, The Shack is the story of a family that has suffered the tragedy of having their six-year-old daughter kidnapped and murdered by a serial killer. They are suffering from all the reactions one can imagine, from intense sadness and guilt to extreme anger with God for allowing this to happen. Mack, the father, finds a mysterious card in the mailbox one day. It appears to be from God and invites him to come to the shack where the last evidence was found of his daughter, a blood-stained dress. When Mack gets there he encounters the Trinity in a Narnia-style adventure that strives to inform about God and our relationship to Him.The author is part of something called the "emergent Church" (or "emerging Church") movement about which there is a fairly lengthy Wikipedia article. This is a postmodernist christian movement which, being postmodernist, is impossible to pin down but is characterised by an openness to various kinds of "spirituality" which are recontextualised for the culture of today. As one would expect, it serenely transcends the old-culture "modernist" characterisation of "conservative" and "liberal".
From what I have seen so far, "The Shack" seems to be emotionally engaging and influential: it is the kind of book which people read and say "it changed my life". However, it plays to the easy, culturally acceptable, rejection of formal doctrine, worship and ecclesiastical structures. Many evangelicals have criticised it because of the way that it treats the bible.
My fear is that among ordinary people it is likely to do some significant harm precisely because it reinforces these cultural prejudices in the context of an attractive story. So some of us are going to need to read it with a critical eye and begin to develop some answers that can challenge those who say that it has "changed my life."
After a few years of the Da Vinci Code, I hope people are reasonably competent in countering the "it's only a work of fiction" argument.