Here’s a book about gunsmithing that you can download as a 40MB PDF file. It’s Advanced Gunsmithing by W. F. Vicery and it was published in 1940.
It’s an old book but I think you may still find it useful and interesting, even if you don’t own a gun, because it describes how to use many of the same machine shop tools and techniques that we’re still using today. I’ve looked at even older books and have been amazed at how little things have changed in the last 100 years, aside from obvious things like the development of carbide cutting tools, EDM and automated (CNC) machines.
3 March 2017: I moved the download link to my Dropbox because the old location has been costing me about $2-3/month for the last seven years. You don’t need a Dropbox account to download it. Just click on the link that says “No Thanks Just Download” or something like that. — Rob