Daniel Newman has a great quote explaining something that is often missed or simply assumed about penal substitution, and which the authors of Pierced for Our Transgressions do, in fact, address. (HT Bish)

“It is on the basis of our union with Christ that our sins are imputed to him and his righteousness imputed to us. Christ is in us and we are in him, indwelt by the Spirit. He was no innocent third party when he suffered, bled, and died on the cross. It was God himself, the Son incarnate as a man, who suffered the just punishment for sins that became his on account of his union with sinners. Our sin must have been imputed to Christ in the first century AD, but God, who is outside time, contemplates in advance the union of his people with him and so the imputation of our sins to Christ and his righteousness to us is just, for in God's mind, the accomplishment of redemption at Calvary and its application to believers at conversion when they are united to Christ by faith and indwelt by the Spirit are a unity. On the cross, Christ died as our representative and substitute. Contemporary Western individualism has perhaps obscured the Bible's teaching on corporate moral responsibility. In Romans 5, Paul teaches that because of the union of fallen humanity with Adam, the guilt and corruption of original sin are imputed to us. There are similar occasions in which people are corporately held responsible for sin in Scripture — Joshua 7 and 2 Samuel 21, as well as occasions where people are corporately blessed as a consequence of the righteousness of others — 2 Samuel 21, 2 Kings 22).”

Continues with "The Atonement - If Jesus Was Not a Sinner, Why Did He Die?"