A BIG subject. Many excellent works mentioned; I wish I had all of them. Here's a few more;
General Subjects
NRA Book of Rifles and NRA Book of Pistols, W.H.B. Smith.
Small Arms of the World; Smith and Smith
A Rifleman Went to War; McBride
Shots Fired in Anger; George.
Special subjects, whatever interests you
System Mauser; Breathed and Schroeder
Mauser Pocket Pistols; Pender
The World's Machine Pistols and Sub Machineguns; Nelson and
Musgrave
Centerfire Pistol and Revolver Cartridges; White and Munhall
History of Modern US Small Arms Ammunition; Hinckley, Woodin and Scranton.
Astra Automatic Pistols; Antaris.
Us Rifle M 14; Stevens.
The Black Rifle; Stevens.
The Pennsylvania Kentucky Rifle; Kaufman.
Single Shot Rifles; More Single Shot Rifles, Still More Single Shot Rifles, Boy's Rifles: Grant.
The Revolver (4 vols), A.W.F. Taylerson
Mannlicher Military Rifles; Scarlatta.
Sharps; Frank Sellers
Revolvers of the British Service; Chamberlain and Taylerson.
Trapdoor Springfields; Waite and Ernst.
Book of the Garand; Hatcher
War Baby, Vols I and II, Larry Ruth.
French Service Handguns; Medlin and Huon
Proud Promise; Huon.
Probably lots more I haven't mentioned.