Book wrote in the Daily Star Sunday: "I've been lucky enough to be invited on some of City's European trips and you see the players up close on those flights.
"I've passed the time of day with Mario and the rest of the boys on those journeys. I see him with the rest of this fabulous squad and you are left in no doubt that all the players love him.
"Mario is as good as gold. You just know that he's going to produce - and we're only witnessing the start of a great career.
"He is only a young lad and can only get better."