There is a wonderful publication put out by the Illinois Secretary of State entitled Origin and Evolution of Illinois Counties. The first few pages of the book contain general information about the counties that were formed and when, while the last few pages indicate who/what the county was named for. But the middle of the book, the bulk of it, are detailed maps that show the boundary progression from 1790 through today. The older county boundaries are shown “on top” of the current county lines to give you a better visual of how the boundaries changed. It also gives you a better understanding of where events may have been recorded at different time periods. This terrific resource is available through the Illinois Secretary of State in a PDF format.