Here is an ongoing list of some fantastic reference materials that I have found helpful. I will be the zillionth to warn you: do NOT eat anything unless you are 100% sure of its identity. Foraging is fun, but for Pete's sake, be smart and be careful.
All that the Rain Promises, and More... by David Arora. The go-to pocketbook guide for mushroom hunting.
Plants of the Pacific Northwest by Jim Pojar. It's hard to find a species of mushroom on a certain species of tree if you don't know what that tree looks like. Start here.
Pacific Northwest Foraging by Douglas Deur. A great guide to the most commonly-found edibles in the Pacific Northwest.
Mushrooms Demystified by David Arora. The most gigantic reference for all things mushrooms. Not a pocket field guide, but keep it at home for more complicated discoveries.