Hello! I consider you to be in the Bluegrass region as a whole. Major regions are what’s mainly used in selecting native plants. Sub-regions, such as The Knobs, are really for if you are looking at rare plants that might not overlap with a major region.
Our article overgeneralizes the regions, as many region borders don’t fall precisely on a county line. In actuality, the lower half of Madison County would be considered in The Knobs region, such as Berea. The upper half, like Richmond, would be part of the Outer Bluegrass. But both fall into the Bluegrass region as a whole. I highly recommend the Kentucky Geological Survey if you’re looking for a scientifically accurate map.
As far as books go, the only one I know of that talks about the Bluegrass region, specifically the Inner Bluegrass, is “Bluegrass Land and Life” by Mary E. Wharton and Roger W. Barbour. The part discussing pre-human settlement is available for preview on google books here, if you don’t want to purchase the entire book.
I hope this helps! Let me know if you have any other questions.