The New York Times review of this book points out the chain that led here. First John James Audubon gave us Birds of America in 1838, then Roger Tory Peterson gave us Field Guide to the Birds in 1934. Now, The Sibley Guide to Birds (published by the National Audubon Society) enters the roll of great natural history books.
This book is often cited as the prime resource for birding for good reason. I use it at least once a week as backyard birder and I see it as a sort of middle ground between Audubon's peerless but sometimes unscientific bird paintings and Peterson's depth of description. I had the Peterson guides as a young birder and what they lack in accuracy of modern taxonomy and range, I still believe they amply make up in behavior, nesting, etc, details. And if one finds The Sibley Guide to Birds lacking in this regard, he has other tomes like The Sibley Field Guide to Birds of Western North America which go into greater depth.
Sibley does a good job of describing why, when, and where you might see birds that deviate from his renditions. Birds are shown on the wing, perched, in different plumages, with sexual dimorphisms, and in characteristic behaviors. These are valuable for identifying birds seen briefly or at a distance; when, for example, you only get the gestalt of the bird: the sum of several vague average differences can together make for a fairly reliable identification.
With this book in hand you'll be half-way to doing just that.