I also use The Standard Directory of Proof Marks; with WWII German Ordnance Codes
by Gerhard Wirnsberger, translated by R.A. Steindler. I have an older edition ofThe Official Guide to Gunmarks; The Ultimate Gun Identification Guide
by David Byron.
There are some cross-reference lists online and in print. Also some manufacturers list serial number ranges by years on their websites (I know Ruger does at least).
Gun Parts catalogs are also good reference books. You may also want to pick up The Standard Catalog of Firearms
These are all good general publications. Quite a few companies have publications dedicated to that particular company and/or firearm (The Book of the Garand, FN FAL, etc). An extensive library is a must. I probably have around 100 books on firearms but I still don't have near enough to look up everything I want to.