For a fascinating overview of this very subject in such a terrible testing ground as was WWI, I reccomend:
DEAN, Bashford, "Helmets and Body Armor in Modern Warfare".
The original book was published in 1920 (I think) and summarizes the conclusions of a commitee at the head of which was Dean himself (by then Arms & Armour curator of the MET), dealing with the analysis of personal body protection during the past conflict in order to make new (theroretically useful) proposals for the army in this field.
Or something like that.
An original is extremely hard to find and expensive, but there's been at least a couple of modern editions in the last few years, and they can be found at a good price.