You can go to amazon.com and look up "graphology" or "Handwriting analysis" and find a lot of books.
You can see some different examples of things like grapho-deck and others at
http://tinyurl.com/afbd3/catalog_af.html#book
Bart Baggett is the author of these.
I do have the grapho-deck that has some samples to show different types of writing traits to look for. Maybe seeing that would help you know where to go from?
I don't know if the books by Bart Baggett would give you the kinds of information you need to figure out if handwriting is by the same person, but they do have some courses for people to become document experts -- to find out if something is a forgery. Maybe you could email them directly and see what they have to say?
Hope that helps. Terri