Some wild plants are easy to identify, like blackberries and apples, elderberries and cattails, even using pine needles to make tea. But if your not sure about the plants then it is best avoiding them all together.
I read a good book once about two women who were kidnapped by Shawnee Indians, in 1755, in Virginia. They escaped and had to live on wild plants and fruits and nuts, as they made their journey back home. The book is a true story. It is called Follow The River, by James Alexander Thom. Anyways, they ate what they found and some of it was good and other things made them violently sick. So it is a good guide in the sense that it shows how to women survived in the wilderness alone with no gear or protective clothing and lived to tell the tale.
Books that are true to life can be a interesting way to understand the need for knowing how to survive in an emergency on the trail.