Pre-pro collectors are a minority in the shot-glass collecting world.  The majority collect modern glasses, of which there are countless hundreds of thousands.   Mark Pickvet has published several books that attempt to grapple with this overabundance by introducing a cataloguing system. 

While not of great interest to pre-pro collectors, the books do contain information about glass history that may be useful.  The values placed on pre-pro shot glasses (called "whiskey samplers" in the books) are considerably higher than realized at auction. 

Shot Glasses: An American Tradition, by Mark Pickvet, 1989.  Antique Publications, Marietta OH 45750.  ISBN# 0-915410-62-1 

The Definitive Guide to Shot Glasses, by Mark Pickvet, 1992. Antique Publications, Marietta OH 45750.  ISBN# 0-915410-90-7

The Encyclopedia of Shot Glasses, by Mark Pickvet, 1998.  Antique Publications, Marietta OH 45750.  ISBN# 1-57080-042-1   

Shot Glasses, by Mark Pickvet, 2004.  Schiffer Publishing Ltd., Atglen PA.   ISBN# 0-7643-2079-3   (read a review)