How to calculate the location of C.G. A.C. & N.P. in static longitudinal stability?
Why assume the c.g. is aft of the a.c. and the tail is providing positive lift?


Maybe the two authors were sick and tired of that old meme that positive longitudinal stability requires negative lift on the tail. This is simply not true and never was, still it is believed like some religion.

What is true is that more static stability needs less tail lift, and many aircraft have so much stability built in that their tail lift is always negative. But with modern flight control systems this negative lift means more drag than necessary. Back in the old days, when the pilot wanted to let go of the stick or yoke in order to read a map or take a leak, a tail downforce was the best way to make sure the aircraft would behave. Still, low and sometimes missing roll stability meant that the pilot could never take his eyes off the horizon (real or artificial) for more than a short while.


  Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?


The position of the center of gravity relative to the wing's aerodynamic center has no significance for stability — all what counts is the center of gravity position relative to the overall aerodynamic center, fuselage and tail effects included. A smaller tail means that for the same stability the tail lift has to be lower, and most aircraft have the smallest tail surfaces that their designers could fit.