Those whom God effectually calls, He also freely justifies; not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for any thing wrought in them, or done by them, but for Christ’s sake alone; nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on Him and His righteousness by faith; which faith they have not of themselves, it is the gift of God. - WCF 11.1
Resources
Justification by John Murray
Of Justification by Thomas Boston
Sola Fide: The Reformed Doctrine of Justification by J. I. Packer
Justification is by Grace Alone by John Calvin
An Alien Righteousness by B. B. Warfield
Justification by Louis Berkhof
Justification - Romans 8:28-30 by Martyn Lloyd-Jones
Double Imputation by R. C. Sproul
Imputation by B. B. Warfield
The Active Obedience of Christ by J. Gresham Machen
More essays on Justification...
Multimedia
Justification Defined - WCF 10.1 (MP3) by Jerusalem Chamber
Justification by Faith Alone (Lecture Series) by R. C. Sproul
Justification by Faith Alone (YouTube) by Michael Reeves
Of Justification - Westminster Confession of Faith 11.1-4 (MP3) by Bill Edgar
Effectual Call, Justification (MP3) by Dr. Michael Barrett
WCF Chapter 11: Justification by Faith (MP3) by Dr. John R. DeWitt
eBooks
The Doctrine of Justification (eBook) by James Buchanan
The Everlasting Righteousness (eBook) Horatius Bonar
The Doctrine of Justification by Faith (eBook) by John Owen
Of Justifying Faith (eBook) by Thomas Goodwin
Vindication of Justification (eBook) by Robert Traill