The Sibley Guide to Birds by David Allen Sibley
I wasn't sure how to choose a non-fiction book about which to write, so I went to my LibraryThing page and pulled up all the books I've ever given 5 stars (the highest possible rating; I only give these out occasionally). Of the non-fiction that appeared there, this is the book I've spent the most time with, by far.
I've used several birding field guides over the years. My first was the Golden Guide, and then Peterson's Eastern. When I got my first National Geographic guide, I fell in love and that quickly superseded even Peterson. And then David Sibley released this guide. From the moment I had it in my hands, I felt like I'd found my One True Love, and I've never looked back.
This guide is beautiful. It has all the basic things I like in a field guide - it's illustrated (I vastly prefer illustrated guides over those with photos, which is why I've never been fond of Audubon guides); the text is clear and helpful, and the maps are on the same pages as the descriptions and color plates. But mostly, I love love love the illustrations. No one artist could ever capture the best possible illustration of every single bird, but I think Sibley comes close. His illustrations tend to match most closely what I see when I'm out in the field. I don't know if this says more about him, or me, but whatever the reason, I adore this book. There are many field guides - many excellent guides, I might add - on the market, but this is by far and away my favorite, and the one I would recommend to anyone who just wants one guide for North America.