Christopher Leahy writes natural history guides, focusing particularly on New England. He writes for both the experienced and novice naturalist. Most of his books are published in the well-known Peterson Field Guide series by Houghton Mifflin. His introductory works include Peterson First Guide to Wildflowers of Northeastern and North-Central North America and Peterson First Guide to Insects of North America. His advanced guides include The Birdwatcher's Companion: An Encyclopedic Handbook of North American Birdlife. His other titles include The Nature of Massachusetts.