Of the books listed on Amazon in FrosSsT's list, I would strongly recommend "Understanding Firearms Ballistics" by Rinker. IMHO this is one of those rare technical books that's written in such a way that a layman can understand most of it, but none of the important stuff is "dumbed down" or disregarded because the author feels it's too complicated to explain easily.
FWIW I've found that many public libraries stock this book.
I would also recommend "Small Arms of the World"; the book was originally written by Joseph W.H.B. Smith but was subsequently updated by numerous coauthors and editors. The latest edition is the 12th edition, which was published in 1983 and is now sadly out of print. Although it's nearly 30 years out of date, and consequently doesn't contain info about the latest "tacticool" weapons, AFAIK no currently printed book covers the history and background of military firearms in as much breadth and depth as this one.
Fortunately, used copies are fairly easy to find on eBay and elsewhere. (BTW check your PM later.