Here is a radio interview with William Young the author of The Shack.
Some of my observations: Young isn't a fan of limited atonement but not a universalist either, he believes in a "real hell." Regarding God the Father being presented as a woman he says God contains source of male and femaleness but says he doesn't want to denigrate the fatherhood of God and concedes God is presented with masculine imagery in Scripture. The interviewer asked about this quote from the book "[God speaking to Mack] Sin is it's own punishment ... I don't need to punish people for sin." In response Young disturbingly denies subsitutionary atonement, saying he agrees with some elements of it but reckons given the theological debate around the doctrine; "[Young speaking] it's much wider then that ... Not necessary to have the Father punish the Son." Annoyingly about halfway through the interview, Young and the interviewer get bogged down in a big disagreement about defining "Christian." They don't delve into much more beyond that.
[I found it interesting when I thought about the interview, how Young distinguishes frequently and strongly between 'sin' and the sinner.]