Armstrong's book is filled with beautiful photographs and covers all the regular species that you will see in Alaska. It is a great introduction to the birds that inhabit the area. Every Alaskan birdwatcher should have a copy.
But, if you are a beginning birder, or new to the Northwest you might need a different field guide. Animals are illustrated with one, two, or three photographs. Unfortunately photographic guides are often deceiving in the field. I recommend you take another good field guide with you.
Each species has 90 to 100 words to cover field marks, similar species, voice and habitat. In some cases like the Black-Backed Woodpecker, only 56 words are used. The writing is great, but the scope of the book does not allow in-depth coverage of each species.
Range finding is limited to a chart with six rows for regions and four rows for seasons. In Alaska, six regions are equal to six average states in the lower forty eight and saying Common, Uncommon or Rare in a region doesn't help pinpoint the species much. A map would do better to give an idea of locations (at least pointing out a species as coastal, or centered on mountain ranges in a given area etc.)
I wouldn't talk anyone out of getting this book, it is beautiful and well written, but if you are planning a birding trip and need to find locations - try West's A Birders Guide to Alaska, and if you are unsure of your ability to indentify birds in Alaska, at least augment this book with the Sibley or National Geographic Guides. If you going to one of the shorebird festivals, definitely get a specialized guide like Paulson's Shorebird Guide.
***** Update *******
There is now a new edition of this book since this review was written, for the most part, the strengths and weaknesses are the same. However, the one thing that has changed strikingly is that many if not most of the photos were replaced and are even better than those from the last edition. A beautiful book just got more beautiful.