Brian,
Aerodynamics for Naval Aviators is a great book and still relevant more than half a century after it's release.
I agree that AfNA was not the source for the classification of slips into "forward" and "side" varieties taught to generations of American pilots.
While I don't know the origin of this terminology, it has appeared in FAA training publications like the
Airplane Flying Handbook and the
Pilot's Handbook of Aeronautical Knowledge for many decades. I've also seen the terms used and the techniques for performing them described in numerous commercially produced flight training materials. Demonstration of a forward slip to landing is still a required TASK in the most current private pilot
practical test standards. No mention of side slips is made in reference to crosswind landings though!
However I'm still pretty sure most slips are slips only because a sideslip angle exists!