It's a UK based guide so, depending where you are, it may not help you much. It's called Food for Free. :)
Food For Free is pretty much considered the gold standard: https://www.amazon.co.uk/Food-Free-Collins-Richard-Mabey/dp/0007183038/ref=sr_1_1?ie=UTF8&qid=1548249135&sr=8-1&keywords=food+for+free
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The SAS survival guide is great but is a bit lacking on the edible plants section so I also have books on edible plants and one on mushrooms which are the same size and fit nicely together in a tin I keep in my bag.
When it comes to the plants I know a lot of it already but it makes for a good aid to identification if unsure. The mushroom book seems essentially if you are planning to forage those given the risks.
Absolutely shocking!
£4.