Encyclopedia to me means very extensive and comprehensive
That sounds like an Amazing plant book I would love to have! What I do own, is very thorough and detailed. I haven't ran across any Like that, but if it exists I'll find it eventually.
Well I have several books that I know of and i understand how you feel about the color photos. There is one book I know that has photos cover to cover around 350 pages handbook sized" called "Medicinal Plants and herbs" and it covers Eastern and Central America. Just as you hinted Amazon.com is a great place to find books.
There is a lady Linda Runyon @ http://ofthefield.com/
She updated her book to have color photos- I haven't Purchased it yet- approx 300 Pages
I found it wonderful in the sense that she covers the most common and has recipes in the back that make the idea of wild foods more practical in mind. Ive found a guy "Wild Man" Steve brill that has an edible wild plant i phone application with color photos for your phone. I think he is also on youtube
Ive found that the more you try to remember plants or identify them eventually you'll realize that they don't have to be in color. At first I didn't think that was possible and those stupid drawings would frustrate me. Now After reading through the color photos for awhile, I go back to the black and white books and I really surprise myself Sometimes. This is after practice but I'm an amateur.