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 Volume I of my eBook on 12 wild edible plants in the Washington, D.C. area (that is, the Eastern United States).  It is 48 pages long with a glossary and information on safe and sustainable foraging. 

Volume I and Volume II (with 12 more local wild edibles) available for purchase in my store by clicking HERE.

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.

Volume II includes Chicory, Chickweed, Red Clover, White Clover, Dame’s Rocket, Dead Nettles, Henbit Dead Nettle, Purple Dead Nettle, Jerusalem Artichoke, Common Mallow, Nettles (Stinging Nettle & Wood Nettle), Purslane, Wild Carrot, and Wild Garlic & Wild Onion.