The Shack is being advertised as THE book that will help you "see God as you never seen him before". (that's an understatement!)
Below is a simple graphic that shows you the unholy trinity at the heart of The Shack.
In The Shack 'god' the father is an African American WOMAN called Papa (a goddess!), 'god' the son is a plain looking Middle Eastern man with a big nose and 'god' the spirit is a small, delicate little Asian girl (another goddess!). The Shack also teaches the heresy of modalism and is chock full of other serious doctrinal errors.
Sadly, Christians are buying this book in droves and claiming that it's changed their lives or opened their mind to new ideas about God. But, this book teaches false doctrine, false theology and a false triune godhead and the author has publicly rejected the doctrine of Sola Scriptura.
This is not the one True God of the Bible. This is an idolatrous picture of God that contradicts what God has revealed about Himself. Christians should not read this book in their quest for finding spiritual truths because the false teaching is so bad and so dangerous that this book can only be classified as 'spiritual poison'.
Heed the warnings of scripture.
2Th. 2:9 The coming of the lawless one is by the activity of Satan with all power and false signs and wonders, 10 and with all wicked deception for those who are perishing, because they refused to love the truth and so be saved. 11 Therefore God sends them a strong delusion, so that they may believe what is false, 12 in order that all may be condemned who did not believe the truth but had pleasure in unrighteousness.