These are excellent books for the beginner and experienced forager. 
I highly recommend having at least one book specific to Wild Edibles and one specific to Wild Medicinals.

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Peterson's Field Guides

By Samuel Thayer

Edible Wild Plants Eastern/Central North America

 Medicinal Plants Eastern/Central North America (3rd)

Western Medicinal Plants and Herbs

Nature's Garden
Guide to Identifying, Harvesting, &  Preparing Edible Wild Plants

Forager's Harvest
Guide to Identifying, Harvesting, &  Preparing Edible Wild Plants

This is an excerpt from my eBook on 12 wild edible plants in the Washington, D.C. area (that is, the Eastern United States).  It is 46 pages long with a glossary and information on safe foraging .  It is available by donation through PayPal (minimum  $1). 
Volume I includes Plantain (Common and English), Docks (Yellow & Broad-leaf), Ground Ivy, Bull Thistle, Burdock, Cattails, Cleavers, Dandelion, Garlic Mustard, Lamb’s Quarters, Japanese Knotweed, and Yellow Wood Sorrel.
I am planning on writing a Volume II with 12 more local wild edibles that will be available late next spring.