Hey Jim;
These are some of my favorites:
-AFTE Glossary
-Firearms Investigation, Identification, and Evidence, 1st ed by J. Hatcher
-Firearms Investigation, Identification, and Evidence, 2nd edition, 1977. Hatcher, Jury , and Weller
-Heard, Brian. Handbook of Firearms and Ballistics, 1st edition, 1997.
-Mathews, J. Howard. Firearms Identification,
These will get you started
Good Luck