The Three Forms of UnityThe Westminster StandardsBooks on the Heidelberg CatechismBooks on the Westminster Standards
Commentary on the Heidelberg Catechism, Zecharias Ursinus,
An Introduction to the Heidelberg Catechism, Lyle Bierma

For Children:

The Quest for Comfort, William Boekestein

For Young People:

Three Men Came to Heidelberg and Glorious Heretic, Thea B.Van Halsema The Heidelberg Catechism, G.I. Williamson

Sermons on the Catechism:

G. Van Reenen
Joel Beeke
Gerrit Kersten
Herman Hoeksema
George Bethune

History of the Westminster Assembly of Divines, William Maxwell Hetherington
A Short History of the Westminster Assembly, William Beveridge
The Westminster Assembly, Robert Letham
The Confession: The Westminster Confession into the 21st Century vol 1, 2, 3; Ligon Duncan
The Westminster Confession: A Commentary, A. A. Hodge (print | epub | kindle | pdf)
Faith of Our Fathers, Spears, Wayne R.
The Westminster Confession of Faith Study Book, Pipa, Joseph A., Jr.
The Westminster Confession of Faith for Study Classes, Williamson, G.I.
Grounded in God’s Word, Hustedt, Dennnis
Truth’s Victory Over Error, David Dickson
The Reformed Faith, Robert Shaw (online | print)
The Larger Catechism: The Westminster Larger Catechism, Vos, J.G.

The Shorter Catechism:

The Shorter Catechism Project
The Westminster Shorter Catechism for Study Classes, Williamson, G.I.
The System of Theology Contained in the Westminster Shorter Catechism, Hodge, A.A. and J.A. Hodge
The Shorter Catechism Activity Book: Learning The Truth Through Puzzles, Marianne Ross