The most comprehensive, useful, practical, portable field guide I've ever found for identifying birds is All The Birds of North America, by Jack Griggs. It's packed full of useful information, wonderfully organized, and easy to use. It's designed for real field use, and it works. Rather than organizing birds by evolutionary sequence, this book organizes them by beak and body shape, which helps avoid a lot of confusion, and makes it especially useful for beginners. I got this field guide when I was just starting to learn bird identification, and it's helped me out so much. Some people might prefer regional field guides - most guides are split into eastern birds, western birds, etc - but I really enjoy having all the possibilities in one field guide. The illustrations are very clear, lively and realistic, rangemaps and information are easy to follow and it's all on the same page - no remembering numbers and names and flipping back and forth for range information.
It's such a sturdy and continuously-helpful book that I actually keep my life list in it, recording the dates and locations of sightings right there on the page, next to the illustrations. This copy of the book's been dragged through hikes and canoe trips and family vacations for almost ten years, and though clearly well-used, it's held up wonderfully, with every page still in almost pristine condition.
Though not a field guide, another book I found very useful when first learning which bird is what: The Birds of Texas, by John L. Tveten. A hefty book, full of gorgeous, clear photographs. It's a great companion to the previously-mentioned field guide, giving me a more detailed and intimate look at the birds of my home state. It covers just about every bird you could see in Texas. It can be great when you're just beginning (pictures and detailed information can be very helpful, and you can really get a feel for what to look for where you live), but it's also great to thumb through every now and then when you're more experienced, to remember all the awesome birds close by you haven't seen yet, or the behaviors you have yet to glimpse.
For mammals, fish, amphibians, and reptiles, I use the respective Audubon Society Field Guide, because I like their clear photographs. I don't use any of these as much as my bird field guide, but they've all been useful when I've needed them.
I've never quite gotten into the maze that is insect identification, but I still have several field guides for bugs in my possession, the most notable of which might be A Field Guide to Common Texas Insects, by Bastiaan M. Drees and John. A. Jackman.
The following list features some of my favorite non-fiction books on the subject of animals and nature. These are books that have left very lasting impressions, and that I would recommend to almost anyone in a heartbeat.
(list format here -- covers below, for the more visual among us)
On Watching Birds by Lawrence Kilham
In the Country of Gazelles by Fritz R. Walther
Eye of the Albatross by Carl Safina
Birdology by Sy Montgomery
Of Parrots and People by Mira Tweti
Alex & Me by Irene M. Pepperberg
The Parrot Who Owns Me by Joanna Burger
The Wild Parrots of Telegraph Hill by Mark Bittner
A Rage for Falcons by Stephen Bodio
The Animal Dialogues by Craig Childs
The Hunting Wasp by John Crompton
Ways of the Ant by John Crompton
The Hidden Life of Deer by Elizabeth Marshall Thomas
Fresh and Salt Water Fishes of the World by Edward C. Migdalski and George S. Fitcher
Arctic Dreams by Barry Lopez
Some of my favorite fiction books about animals:
Black Beauty by Anna Sewell
White Fang by Jack London
The Call of the Wild by Jack London
Bunnicula by James Howe
The Desert Pool by Guy Chalkley
Bambi by Felix Salten