Wild Harvest: Edible Plants of the Pacific Northwest by Terry Domico
This is the only book I know of that specifically deals with edible plants in the North West. You can get a copy of something like The Forager's Harvest but it will be less useful.
Also look for local survival/outdoor schools. They may teach edible plant courses. Check with your local Park Service. Ours does a free Edible Plants nature walk. Your local Extension Office may have additional information as well.
