Nearctic Wasps of the Subfamilies Pepsinae and CeropalinaeBy Henry K. Townes
Smithsonian Institute Press (Bulletin 209), 1957
Cite: 61576 with citation markup [cite:61576]
This is the literature I'll be citing in my guide pages for Pepsines and Ceropalines. It obviously doesn't cover some species described after '57 but it's still the best comprehensive study. It has some really good keys. Might be hard to buy but most large university libraries should have it.