If you are in the Pacific Northwest, and you want a book with a lot of plants, go for Plants of the Pacific Northwest Coast, by Jim Pojar and Andy Mackinnon. I call it Mac and Poj (gotta eat up that knowledge). This has a ton of western Washington, Oregon, and B.C. plants with some general uses. Easy to carry in the field, color pictures, ecology, etc. If you are east of the Cascades, there is a book with the same format as Mac and Poj, called Plants of Interior British Columbia, which also extends south into Washington and Oregon. I do not know the authors.
If you plan on using these plants for medicine or food, I recommend getting books that specialize in edibility and/or medicinal qualities of plants. Michael Moore's Medicinal Plants of the Pacific West has medicine recipes for a big chunk of the plants up here. Janice Schofield's Discovering Wild Plants is another good book for edible recipes. The downside of her book is the quantity of plants included. She reports on major edibles and gives each plant at least two pages, with drawings, photos, and even short histories.