It's very useful.  I have a copy from my days on the show circut.  It might help when the dragon is walking on flat ground (dressage is done in a flat arena) but I'm not sure how much help it will be when the dragon is flying.  The wings and their muscle movements might change things a bit but it's a good start.

The more research you do, the better so don't take my comments as negative.  I'd buy them.