This is a relatively new series, with additional volumes appearing every few months. Apparently still to come: western Canada and Hawaii. Each volume is subtitled "Nature Adventures in Parks, Preserves, Forests, Wildlife Refuges, Wilderness Areas".
These are invaluable references for a traveller with an interest in natural history. Each takes a major region of North America and methodically covers it, national parks, state parks, wildlife refuges, and any other significant natural area. The maps and photography are up to National Geographic's usual high standard.
My only real criticism of the series is that they seem a bit heavy on birdlife (a common obsession among outdoors people), and a bit lacking on rare plants and notable trees and forests. The index lists only people and places, not species, making it hard to hunt for a place to see, say, an uncommon cactus or a rare bird.