I first heard of The Shack when family members mentioned that it discusses the Trinity. Just a tip: When a popular work discusses the Trinity, warning flags should go up. Historically, attacks on the Trinity have been at the core of all sorts of heresies.
One of the chief heresies of the last century is feminism. While feminism may seem spent as a political phenomenon, its errors have invaded the church so deeply that they are unrecognized. Feminists hate the doctrine of the Trinity, not only because it speaks of Father, Son, and Holy Spirit as “He,” but also because it is a model of godly submission. The Son submits to the father and the Holy Spirit proceeds from the Father and the Son in the same way that the church submits to Christ, wives submit to husbands, children obey parents, slaves obey masters, etc. (Ephesians 5:22-24, Ephesians 6). The head is then called to love the one submitting.
Here’s a helpful article on the Trinity.