Kellogg didn't originate hydrotherapy, but his meticulous observations and endless experimentation helped refine the method and put it on the medical map. Then he taught G.W. Abbott, who standardized the subject and wrote probably the most complete manuals on the technique.
These books are good, but sometimes a bit too good. You can find some very practical--and a lot simpler--instruction on YouTube. Just search for "hydrotherapy," or "hydrotherapy adventist" if you want to avoid some of the weirder stuff out there. For massage, try "massage class" "adventist." There's not much there, but that search prompt will cut out all the "massages" given from the pulpit.