The Baltimore Catechism
Last year I wrote about Doctrina Christiana, St. Robert Bellarmine’s catechism for adults. Though excellent, it’s also rather short. Not that a catechism should go into great detail on every point, since it’s intended as a brief introduction to Christian doctrine, primarily stating what the Church’s main doctrines are and not a full explanation, but one can easily think of enough additional questions after reading it that many readers would benefit from something longer.