I haven't seen Forrest's book, though I saw some of his Master's thesis. He believes (or believed), if I understand his thesis, that the choreography of Morris was strongly influenced in the 1700s by the choreography of another dance form known as the Matachin. I don't know what the scholarly reception of this thesis has been.

A good source of what is known about the history of morris is Kieth Chandler, "Ribbons, Bells and Squeaking Fiddles"-The Social History of Morris Dancing in the English South Midlands, 1660-1900, Hisarlik Press, 1993.

T.