Lots of places to look at old guns sorted into schools, such as here in the museum or the CDs that the KRA puts out, but if you are looking for a guide to show you what to look for:
Kaufman's The Kentucky-Pennsylvania Rifle has an overview that is very simplistic - just some stock profiles of the major schools without any more detail - but might provide a useful starting point and has the virtue of being relatively inexpensive.
If you are serious about it, though, you should probably get a copy of Joseph Kindig's Thoughts on the Kentucky Rifle in its Golden Age - Kindig was the guy that originally started trying sort different makers into schools and figure out how they related to each other instead of lumping everything into "Kentucky Rifle." The newest edition with the color supplement runs about $125 new, but you can find the older editions used for much less on bookfinder.com or other sites if you hunt around and are patient.
