Warblers occupy a special place within the imaginations of North American birders. Warblers appear in a diverse array of colorful plumages, but they are not just eye candy. Their elusive habits and complex plumage makes finding and identifying them a satisfying challenge. Most birders have only a few weeks to see the vast majority of warblers in fall and spring, so there is little danger of their appeal wearing off, as sometimes happens with colorful resident species. Even after birding for ten years, I still have not mastered every plumage for every eastern warbler, and there are some species I still have not seen (like Kirtland's Warbler) or only seen once or twice (like Mourning and Connecticut Warblers).
A few features make The Warbler Guide distinctive. The most obvious at first glance is the sheer amount of visual information presented for each of the warbler species. (You can see a sample species account in this pdf.) For some species, the accounts are broken into drab birds vs. bright birds or fall vs. spring. To me, the accounts emphasize the best of the "field marks" vs. "whole bird" methods of identification. While it checks off diagnostic features (what might be called "field marks"), it goes beyond those to show how each bird appears from numerous angles. The photos also show characteristic behaviors and postures, which can be useful evidence in differentiating one species from another.
Another is that it uses a system of symbols and abbreviations with color coding to convey information about each bird. This may appeal some readers, but I prefer that a species account tell me the information in words rather than send me off to some key to find out what the symbol means. The guide also includes a quick finder that covers many angles and plumages. While useful, it starts on page 100, which makes referencing it less quick than it might be if it were placed near the beginning or end of the guide (as the quick finders are in the Crossley ID Guides). The publisher has helpfully provided PDFs of these quick finders on its website, for anyone to reference.
More important is the use of sonograms to represent warbler songs and calls. Sonograms were first introduced in printed field guides with Birds of North America: A Guide To Field Identification by Chandler Robbins, Bertel Bruun, Herbert Zim, and Arthur Singer, but they never caught on enough for authors and publishers of other birding field guides to feel the need to follow suit. Instead, it has been standard for field guide authors transcribe sounds into phonetic sounds, like kek-kek-kek-kek, or mnemonic phrases, like sweet-sweet-sweet-a-little-more-sweet. The trouble with the latter approach is that not everyone hears bird sounds the same way, and mnemonic phrases often do not the rhythm of the song they represent. Sonograms offer a more accurate way to represent bird sounds. However, sonograms take some skill and practice to interpret correctly. To that end, The Warbler Guide includes an extensive guide to sonograms and learning bird sounds in its introduction.
The Warbler Guide by Tom Stephenson and Scott Whittle is an innovative guide for learning and identifying North American wood warblers. It stands out from existing field guides, particularly in its inclusion of sonograms for every species. Any birder with an interest in warblers will want this guide as a stepping stone to more advanced identification skills.