Here is a sampling of some of my favorites:
Didache, chapters 2, 7-10 On abortion, rite of baptism, fasting on Fridays, the Eucharist.
Clement of Rome, To the Corinthians, chapters 42 & 44. On the orderly succession of bishops.
Ignatius of Antioch, To the Romans, Introduction. On the primacy of the Church of Rome.
Ignatius of Antioch, To the Smyrneans, chapters 6-8. On the Eucharist, the bishop, and the Catholic Church.
Martyrdom of Polycarp, chapters 16-19. On Polycarp’s death and the veneration of his relics.
Justin Martyr, First Apology, chapters 61, 65-67. On baptism, the Eucharist, Sunday worship.
Irenaeus of Lyons, Against Heresies,
Book 3, Chapter 3, Paragraphs 1-3. On the necessity of being in communion with the Church of Rome
Book 3, Chapter 22, Paragraph 4. On Mary, the second Eve.
Book 5, Chapter 19, Paragraph 1. On Mary, the second Eve.