Not particularly a book for the Pacific Northwest per se. However, we picked up this guide, as well as Peterson Field Guide to Medicinal Plants and Herbs of Eastern and Central North America, when we spent a little time in Northwest Florida and found that many of the plants in these books also grow out west. And surprisingly, a lot of those plants were not listed in our western references! This just goes to show that it is important to build out your library.
Highly recommended if you plan on spending time foraging in the Central and Eastern US, but also useful for other parts of North America as well.