Didache — Among the oldest Christian documents outside the New Testament — moral catechism, church manual, and the earliest surviving eucharistic prayers.
Marcion — Fragments and reconstructions from Marcion of Sinope, whose radical theology forced the proto-orthodox Church to define its own scriptural canon.
The Shepherd of Hermas — One of the most popular texts of early Christianity, read as scripture by Irenaeus and Origen, presenting visions, commandments, and parables of a freed slave guided by an angel in the guise of a shepherd.