The Didache: A Window on the Earliest Christians

This is a short book by Thomas O'Loughlin discussing (a bit tediously, but in a generous spirit that honors how tradition changes) what appears to be a late first century "teaching" -- a sort of manual for new Christians.  Pagels cited it in her book on Revelations and it is a useful read along with NT writings (Acts also speaks of "the Way" while late epistles have various similar rules for the long haul) to help understand early Christianity.