Barge cement is an industrial-strength contact cement. Shoe makers use it a lot, I'm told. (Barge is the brand name)
It will glue virtually anything to anything, and is about the most toxic substance I've ever been around. I think that for most puppet building projects it's overkill, particularly with the newer adhesives on the market. I've found weldwood contact cement to be every bit as durable and not quite as toxic to deal with. Either way, breathing protection is ESSENTIAL, not optional. Even if you're outdoors in a "well-ventilated area" you mustn't take chances with your health!
As to the book, I don't think that any "secrets" were mentioned, per se, but close observation of the workshop photos might show you some of the materials used, like barge cement. (Which, by the way, comes in a rectangular metal can with a brush-cap in the corner Look on page 55 at the extreme righthand edge of the photo. Also page 117 in the picture of Rollie Krewson with the hair dryer). Probably the things being referred to as "secrets" might be things like the foam ribbing shown on page 44.
Again with the advances in materials over the past 25+ years, you probably won't find and "holy grails" of puppet building here, but it sure is fun to see how they did it all!