Thank you very much for your reply.
I do understand it in a mathematical way. Now if the tail provides a nose-up or nose-down stabilising effect depends on the CG and the contstruction of every aircraft. 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 (wether the CG is at AFT or FWD limit) ?