Early Church Writings
The Apostolic Fathers – Kirsopp Lake (Volume I)
The Apostolic Fathers are a collection of the earliest non-canonical orthodox Christian writings, Volume 1 includes 1 Clement, 2 Clement (first recorded sermon), Ignatius, Polycarp, the Didache, and Barnabas. Highly recommended reading, not too difficult.
The Apostolic Fathers – Kirsopp Lake (Volume II)
Includes the Shepherd of Hermas (long, but highly regarded by the early church), the Martyrdom of Polcarp, and the Epistle to Diognetus.
The Ante-Nicene Fathers: The Writings of the Fathers down to A.D. 325
A classic 10 volume collection of all the early Christian writings prior to the Nicene Council. It includes the above mentioned Apostolic Fathers as well as Justin Martyr, Irenaeus, Clement of Alexandria, Tertullian, Origin, and others.
Collection of Early Church Fathers (CCEL)
Another resource with the Ante-Nicene Fathers, but also includes the Post-Nicene Fathers.