1) American Military Shoulder Arms, Vols. 1-3 by George Moller (especially vols. 2 and 3)
2) Firearms from Europe: 2nd Edition by James Whisker
3) Confederate Rifles and Muskets by Murphy and Madaus
4) The English Connection by Pritchard and Huey
5) Entrepot by C.L. Webster III
6) Confederate Musketoons and Carbines by Murphy and Madaus
Those are generally my go-to gun books, although Entrepot focuses on imports as a whole rather than just arms. I'd like to see what books others have in their reference library.