How about an authoritative guide to the native plants of a region? One that gives you a key, scientific, and common names, as well as some illustrations to show you what they look like. I don't know what the proper book for IL would be, but for the PNW, it is
Hitchcock, Leo L., and Arthur Cronquist, Flora of The Pacific Northwest. An Illustrated Manual. Or you could go for something more gaudily illustrated, like the new two volume Flora.