Small Catechism
Luther's 1529 handbook on the essentials of the faith (the Ten Commandments, the Creed, the Lord's Prayer, Baptism, Confession, and Communion), written in plain words for anyone just starting out. It has always worked equally well read alone or read with someone else. Still the standard entry point into Lutheran teaching.
In The Confessions — The documents collected in the Book of Concord.