Some of the reference books I use:

 "A Catalog History of Schrade Knives (Uncle Henry & Old Timer)" by Robert Clemente

"The Collectors Guide to Switchblade Knives"  by Rich Langston

"Levine's Guide To Knives and their values" Fourth edition by Bernie Levine

"Antique American Switchblades" by Mark Erickson

"George Schrade and His Accomplishments to the Knife Industry" by George M. Schrade

"American Premium Guide to Pocket Knives & Razors" by Jim Sargent 1989

"Big Book of Pocket Knives" by Ron Stewart and Roy Ritchie

"Official Price Guide to Collector Knives" by C. Houston Price 13th edition

"Keen Kutter Pocket Knives" by Alvin Sellens