DBBill,

Christopher Austyn has an authoritative book on British engraving styles and Tom Turpin has a nice book on US engravers. I have also seen Mario Abbiatico's book (please correct me if my spelling is wrong as I don't know Italian and am typing from memory from many years ago) "Grand Incisioni su Armi di Oggi" though from what I know only one edition was brought out and it is now a highly sought after collectible.

You could also do Google searches for the great names in engraving like Harry Kell, Winston Churchill, Raschid El Hadi, Manrico Torcoli, Rachel Wells etc .

Good hunting!