Kaufman Field Guide to Nature of New England I have used this book more than any other field guide since moving up to New England. It has a bit of everything from birds and flowers, to geology and seashells, from constellations and insects, to trees and vertebrates. It is small enough to carry in a backpack, and has great photographs. I used it so much in summer camp that my campers started grabbing it to answer questions they had. I hope Kaufman makes more regional field guides because this one is fantastic.
Sibley's North American Birds The best illustrations of important id marks.
The Crossley ID Guide, Eastern Birds- A wonderful collage of bird photos showing birds at different angles and color variations. It is too heavy to carry in the field, but a good book to study before and after going birding. Also a great reference for painting birds!
It's not really a book, but it is a guide to feather identification. Fish and Wildlife Feather Atlas
Kaufman Field Guide to Insects of North America Photos of the bugs or at least representatives of most bugs you'll find around the yard and in the woods. Text is well written with facts and interesting comments.
Caterpillars in the Field and Garden; A field guide to the Butterfly Caterpillars of America I've used this field guide several times with success. I wish it showed more moth caterpillars but it is a good field guide.
I received Tracks and Sign of Insects and Other Invertebrates for my birthday. The authors have written and compiled photos on everything a bug and other creepy crawly might leave behind: eggs, molts, leaf damage, galls, tracks... This book is published by Stackpole Books and the publisher has several other great reference books including Bird Sign, Mammal Tracks and Sign, and the next book listed below on Animal Skulls. These books are thick and would be hard to carry in the field, but I think they are wonderful.
Animal Skulls: A Guide to North American Species - I love this book. It has beautiful drawings of skulls with key identification markers highlighted. It has measurements for separating similar species, and includes several bird and reptile species. The first 4 chapters are wonderful for helping to understand the different bone structures, kinds of teeth, and marks on the bones. I learned a lot from this book.
Smithsonian Handbooks Rocks and Minerals Has good photos and a nice rock identification key in the front. It is for less advanced geologist.
Golden Guide Seashells of North America Has interesting information about the snails and clams so you can get an idea about the animal who lived in the shell.
Cloud spotter's Guide A really fun read, and an introduction to a new hobby, cloud spotting!
Peterson's Reptiles and Amphibians- Side by side comparisons of animals, with detailed info in back and
I don't prefer many of the Audubon Field Guides, but I do like their Tree Guides. There's one for the Eastern US and another for the West. I use them a lot at work when kids bring in pine cones, and such. The tree guides are well organized and easy to use which is important when you're working with kids.
Mushrooms Demystified has everything you need to start identifying mushrooms: dichotomous keys, pictures, and lots of tips. I think it weighs about 5 lbs. Now all we need is a little rain so mushrooms will pop up.
I made links to Amazon just so you can check out the books and see a picture. I am not in any way recommending that you buy from any particular company. I don't get anything when you use the link!