For the average layperson, the Catechism of the Catholic Church is where I’d start. It is the authentic teaching of the Church, for the Church in its modern context. I’d also keep a good quality Bible next to it while I read it.
I think this is the most prudent reply.
Some of the material I’ve seen recommended on this thread, such as the writings of St. Augustine, are wonderful, but not all of what he teaches is “canonical”. For example, he believed in double predestination, which the Church repudiated definitively at the Council of Trent.
Similarly, the Vatican II documents must be read with discernment, and in the context of the Church’s teachings. Reading them out of context has led to a lot of errors in both directions.
The Early Church Fathers are a timeless treasure, but again, must be read against their historical background to avoid falling into error.
The Catechism is the best “framework” from which to begin exploring these other priceless resources, as it provides a clear “anchor point”. Next would come the New Testament, particularly the Gospels, Acts and Epistles. (Revelation / Apocalypse should be read only after one is well grounded in the rest, and after a careful study of the Old Testament, to avoid falling into various theological errors, especially the dispensationalist heresy and hyper-Preterism.)
Of course, for specific issues, there are also specific encyclical resources that are of value,
provided they are read against the above background:
Biblical scholarship:
Providentissimus Deus (Pope Leo XIII),
Divino Afflante Spiritu (Pope Pius XII) and
Dei Verbum (Vatican Council II)
Science and religion / faith and reason:
Humani Generis (Pope Pius XII),
Fides et Ratio (Pope John Paul II)
Marital and sexual ethics:
Humanae Vitae (Pope Paul VI)
Modernism and liberalism:
Pascendi Dominci Gregis (Pope St. Pius X)
Internet resources, though easy to access, are still a bit iffy to an old-timer like me, who prefers reading stuff in print. (By Internet resources, I mean online sites with apologetic essays, blogs, private speculations, “traditional” sites, and so on. The Vatican website, or sites which host the Early Church Fathers such as New Advent, are equivalent to print resources in my book.)
