Penal Substitution

Dr. Albert Mohler provides a primer on penal substitution, harkening back to a J.I. Packer lecture 30 years ago.
Speaking of which, Crossway has just released Preaching the Cross, a volume with contributions from Mark Dever, Ligon Duncan, Albert Mohler, C. J. Mahaney, John MacArthur, John Piper, and R. C. Sproul.
From the Publisher’s Description:

Proclaiming the gospel is without a doubt the most important task of pastoral ministry, yet often other seemingly urgent activities can obscure it. From time to time all pastors and preachers need a fresh reminder of the primacy of the gospel.
Preaching the Cross does just this. It is a call to preaching the gospel as the center of pastoral ministry. This volume showcases an unprecedented combination of pastors representing a variety of evangelical traditions. Though they may differ on some secondary points of church practice, they all enthusiastically celebrate the centrality of the cross of Christ.

Note: This book is available directly from Crossway, although apparently Amazon.com is not yet listing it.