Pre-pro collectors are a minority in the shot-glass collecting world.  The majority collect more modern glasses, of which there are countless thousands or probably hundreds of thousands.  Mark Pickvet has published three 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 is  useful.  The values placed on pre-pro shot glasses (called "whiskey samplers" in the books) are considerably higher than realized at auction. 

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

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