For me, forgiving people isn't all that much about letting them off the hook or even saying it's okay what they did.
I forgive for my own benefit. It means I'm no longer hanging on to what happened and allowing it to continue to keep on hurting me. I'm letting it go and letting go is just allowing it to be. It allows me to move on.
If you're referring to The Shack by Wm. Paul Young, I liked it and have Cross Roads here (by the same author), but haven't started it yet.