The Outdoor Pocket Bible: Every Outdoor Rule of Thumb at Your Fingertips by Paul Jenner
This book features information on all you need to know when you're out walking, camping or on the beach, alone or with the kids. From what weather is coming to how to light a fire in the rain, which animal's tracks you're looking at to estimating how long it will take you to climb the hill ahead of you, this book should live in the pocket of your walking jacket permanently, and will be referred to endlessly.