Foraging is NOT looking at a plant and deciding if it's edible, nor is it looking in a book at a plant and then going looking for that plant. It's not possible to learn all the plants and it's not possible that all the plants will be in the area you forage.
Foraging is about confidently identifying some edible plants.
The two main components of this process are slow and steady learning coupled with an understanding of habitat and how plants and fungi feature and interact there throughout the seasons.
I recommend not buying field guides initially. Books are good for reference, but overwhelming in the early learning phase.
Set realistic goals based on the amount of time you spend outdoors.
Try something like this: http://blog.artandscienceofsurvival.com/p/free-wild-food-natural-resources-course.html
When you get a bit more confidence, take yourself on a Fungi Foray, they are always good fun.
Watch out for Wild Food courses, especially those that lay claim to you "learning 30 plants". Nobody learns 30 anything in a couple of hours, let alone remember it a few months later.
Get involved with your local Bushcraft community, they are always good for knowledge sharing. Try some of the groups on Face book like Wild Food (https://www.facebook.com/groups/189368870195/).
Good luck, take it steady and have fun.
Steve (pro survival and wild food instructor) Marvell