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.