The Shack is a beautiful story of forgiveness and redemption (and the incomparable nature of God).
The forgiveness that 'exceeds all bounds' belongs, I believe, only to God. There is an achievable level of forgiveness, however, of which most humans believe they are incapable. Opening ourselves to that possibility, while most difficult, is the beginning.
Like ArtFriend, I believe what forgiveness brings to the forgiver far exceeds that what is brought to the forgiven.