Laine Didier, Diffusion Histoire Et Documents 1998.

Same guy much the same data, less expensive and more available then one of the seminal works in the field. Still far short of the first book and all the text and data are done in French. Of course…. not so many nice pictures but a nice evaluation of relative rareness. This is more important than you think.  Really shows what helmets are hard to come by.  That is in no other source that even attempts this. Also relative rarity is not combined with reserve status so only active relationships.  I like this but the binding is pretty lousy-- actually it is far worse than lousy.