A great firearms guide for writers
I've noticed that there are more than a few stories, threads, and reviews, on this site that involve questions about firearms.
I'd like to suggest that anyone in need of really good technical information on this subject pick up Boston's Gun Bible by Boston T. Party. It is actually about the size of a Gideon Bible and the author answers just about every gun question the average person is likely to have (which gun would be best for X purpose, proper calibers, etc.).
The author is a bit of a Libertarian nutter, and he doesn't hold back on political commentary, but his weapon info is completely indispensable. If anything, his commentary will give an unsympathetic reader insight into the beliefs of a die hard Second Amendment advocate.
The best way to learn about guns is to hang out with gun enthusiasts, but if that option is unavailable, you really can't do much better than this book.