Oh, you and your Little House on the Prairie...(hee, hee), but seriously, what fond memories I have of being at your house while you read a chapter to the girls each night, for the first time in their young lives, and they listened with such care to the story of Laura and her family's daily adventures. And how excited they were waiting for the next night so they would hear more. 
Yes, "The Last Child in the Woods" is a must read, in my opinion. Growing up on a farm, that also had many acres of wood, my childhood friends and I were able to spend entire days (packed lunches and all) completely free to enjoy all that nature had to offer us each day. And each day was unique in the very best of ways. It is my opinion, (along with a few Native Americans I know), that a kid has not lived his/her childhood unless they've broken a limb falling from a tree. I'm not wishing for this be any means, for my own kids, rather making a point. 
Unstructured time IS essential to our very health. And, unstructured time with a cat, dog, or a horse...all the better! Indoors or out, if anyone infringes upon my "free play time" they'll be sure to see a bear. I protect my kids free time just a fiercely. There is enough time for the "should dos" in life.