Thanks for your interest in a follow on to The Shack. You might try Wm P Young's second novel, Cross Roads. It is very like The Shack in that the central character experiences sound kind of supernatural encounter with the three persons of The Trinity expressed in very different forms and with different personalities. (Why change a winning formula?) The outcome is life-changing and not just for the main player in the drama. You might also try Michael Neale's The River.