Reddit mentions of Heidelberg Catechism: Traditional Language

A good work to start with on monotheism in general is The Experience of God: Being, Consciousness, Bliss by the Eastern Orthodox writer David Bentley Hart. For general Christian concepts, you have made a good start with C. S. Lewis.

As for individual denominations, some good short pieces of writing for a concise overview of what the largest denominations believe are the Catholic Creed of Paul VI, the Lutheran Augsburg Confession, the Reformed Westminster Confession of Faith, and for the Orthodox, I suppose, the anathemas proclaimed in the Service of the Triumph of Orthodoxy. Some good books to explore would be The Catholic Catechism, The Orthodox Way, and the Heidelberg Catechism.