Susan - I contacted the Heard Museum here in Phx. They do not know of
any books written on the subject. It did occur to me that you might
want to look for portraits & photos from the period. They probably
won’t show much detail (unless you get lucky with the horses head
framed right). If you have two good pictures as reference, the other
photos might help in extrapolations and alternate styles. Good luck!
Gryphon Song Creations