Justification Defined - WCF 11.1

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

The Reformed Doctrine Touching the Justification of a Sinner by William Perkins

Justification is by Grace Alone by John Calvin (see On the Sixth Session of the Council of Trent)

Acts of the Council of Trent with the Antidote 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