Hi,
This is my first post here, and I couldn't resist responding to this question.
I would buy both of Samuel Thayer's books, Nature's Garden and The Forager's Harvest. I've never seen better books on
wild edibles. I don't remember which book, but one of them lists each state and gives a percentage of how relevant his books are to each one. He is in Wisconsin, and I'm pretty sure Illinois is if not 100% relevant than in the upper 90's.
Each book profiles 40some plants and gives all the information you would need for identifying, harvesting, processing, preparing, and enjoying any edible parts of each plant. There is an entire chapter devoted to each plant and multiple color photos to help with IDing. He only lists plants that he has extensive
experience with and considers not just edible, but delicious. He will also say whether or not certain plants are worth the time and
energy you'd have to go through to harvest and prepare them. There are many instances where he strongly disagrees with some conventional wisdom about certain plants, and is not afraid of letting you know where he stands on certain issues. Even if you're not ever going to harvest some of the plants, it's still a lot of fun to read what he has to say about them.
Anyway, I could go on for a while about how great these books are. I'm sure there are other good books out there as well, but I'd be shocked if they were anywhere near as comprehensive as Thayer. You can get them on Amazon, but I got mine from abebooks.com. I think they also have them on betterworldbooks.com, but I don't know which site would be cheapest.
Hope that helps!
-WY