I think the list below is an all inclusive list of the suggestions that I received; however, if I forget to include something it is not intentional.
George Kaas' headstamp guide
Firearms Guide (latest edition)
Star Firearms
Astra Automatic Pistol
History of Smith & Wesson (Roy Jinks)
Smith & Wesson 1857-1945 (Roy Jinks)
Jack First catalogs (parts diagrams)
Cartridges of the World (10th Edition)
Shooting Incident Reconstruction (Luke C. Haag)
AFTE Journals
Shot Show literature
Firearms manufacturer's catalogs
Ammunition manufacturer's catalogs
Numrich Gun Parts Corp. catalog (parts diagrams)
Gun Digest books of firearms assembly/disassembly for the various types of firearms
Hatcher's Notebook
Firearms Investigation, Identification and Evidence (Hatcher, Jury, & Weller)
The Identification of Firearms (Gunther & Gunther)
The Handbook of Firearms and Ballistics (Heard)
Standard Catalog of Smith & Wesson (Jim Supica & Richard Nahas)
The Blue Book of Gun Values
Reloading manuals that include instructions for downloading (such as Speer)
Gunshot Wounds (Vincent DiMaio)
Firearms Identification (J. Howard Mathews)
Firearms, Toolmarks and the Striagraph (Davis)
Like I said, I believe I included all of the suggestions that I received when I asked the same type of question. Anything that may have been inadvertently left out is not intentional. I hope this helps some.
Richard