If I understand well, for the Oxford POF book it's just a matter of presentation that the Aerodynamical center of the wing is always in front of the CG
That is correct.
Keeping it to a simplified theory you could state, that an aircraft is stable in pitch if the wing loading of the forward wing is higher (in the mathematical sense) than the wing loading of the aft wing. If the Aerodynamical center of the wing is aft of the CG, the aft wing loading is negative, so it is definively smaller than the one of the front wing. So as long as the rear wing is large enough, it is OK if it produces some lift there, so the Aerodynamical center of the wing might be in front of the CG. For many high performance gliders this is the case, as large (negative) lift on the aft wing of course means additional drag.
If you make the aft wing even larger than the front one, you end with a canard configuration, which is stable in pitch if the wing loading of the canard is higher than that of the wing. In that case you can even use absolute numbers, as in any situation both wings will provide lift.