For the Revolutionary War, go with Neumann & Kravic's Collector's Encyclopedia of the American Revolution.
If you're looking for a book to help identify relics of the Civil War period, a good starter would be Howard Crouch's Civil War Artifacts.
For general relics of the mid to late 1800's, check out reprints of old Sears, Roebuck and Montgomery Ward catalogs. Reprints of the old Bannerman military surplus catalogs are a goldmine of information, too.
If you're interested in certain types of relics— for example, buttons or belt plates— then there are entire books devoted to those. Just let us know what you're looking for.