Princeton University’s Index of Christian Art has prepared an excellent annotated bibliography of print and electronic resources related to early Christian art—a field that is greatly in need of further study. Check it out here.
And while you’re in an artsy mood, download a copy of one of the fine works included therein: Walter Lowrie’s Art in the Early Church (New York: Prometheus Books, 1947).
It is nice they added these works digitally, the Museum of Modern Art is using the same process.