This book is by no means exhaustive. It's more like a casual conversation with someone who has been camping for decades: they might still cling to techniques and gear that newer innovations have obviated, they could prefer different camping activities than you (Cliff likes canoe voyaging and it shows), and they just might give you something to think about.
I wouldn't consider it camping's top secrets. However, I'm hopeful you'll find a nugget or two.
Here's my secret that wasn't in the book: If you backpack with an internal frame pack, don't put your sleeping bag into a stuff sack. Instead, open it up and use it like packaging material to fill the voids between your mess kit, food, tent, and whatever else you're carrying. Now nothing is going to jingle and your pack will feel like a giant pillow on your back.