Newberry Library hosts this wonderful tool on their website for tracking county boundary changes. The "Atlas of Historical County Boundaries - A reference work designed to provide information about the creation and boundary changes of every county in the United States, from the earliest county creation in the 1600s to 2000." Choose your state, then select the year on the right. County lines are shown overlaying current county lines, similar to William Dollarhide's "Map Guide to the US Census" for those of you familiar with that wonderful book.
The best part is that this interactive digital map is completely free, courtesy of the folks at the Newberry Library.