Belgic Confession
Heidelberg Catechism
Canons of Dordt
Belhar Confession

Form for the administration of Baptism
Form for the administration of the Lord’s Supper

Directory for Public Worship
Directory for Family Worship

The Catechism in a small booklet, Soft Cover Gift Pocket-Size Edition, in shortened Questions & Answers

Zacharias Ursinus, Commentary on the Heidelberg Catechism, online | print | kindle

What was the history surrounding the writing of the Catechism?

An Introduction to the Heidelberg Catechism, Lyle Bierma
The Heidelberg Catechism: A Study Guide, by G. I. Williamson
The Theology of the Heidelberg Catechism: A Reformation Synthesis, by Lyle Bierma
Comforting Hearts Teaching Minds: Family Devotions Based on the Heidelberg Catechism, by Starr Meade
A Faith Worth Teaching: The Heidelberg Catechism’s Enduring Heritage, by Jon D. Payne & Sebastian Heck
The Church’s Book of Comfort, by Willem Van’t Spijker
The Lord’s Supper and the ‘Popish Mass’: A Study of Heidelberg Catechism Q&A 80, Cornelius Venema
Living and Dying in Joy: A Devotional Guide to the Heidelberg Catechism, Cornelius Vonk

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
Gerrit Kersten
Herman Hoeksema
George Bethune
Theodorus VanderGroe