I have used Amazon books for personal use and for school.
Kindle Books
As a teaching professional, I refer to this book on occasion to make sure that I am thinking correctly. I usually am! Great instruction for teachers and solid drills.
PGA Teaching Manual: Art & Science of Golf
One of the best instruction books written. Harvey was the teacher of many pro golfers, and his methods still hold weight today.
Harvey Penick's Little Red Book: Lessons & Teachings
I love these field guides. I got my first one in 1973 while in high school. I do a lot of bird photography, and I use this as one of my resources.
National Geographic Field Guide to Birds of North America 7th edition
This is a fun,tongue-in-cheek golf instruction book. Leslie Nielson takes you on a playful journey.