To effectively throw deep, besides having the arm, you have to stay in the pocket. Book bails with virtually any inside pressure. He doesn't subtly slide as quality qbs might do, he bails. And it is hard to throw downfield with his arm when he can't set his feet. So the modus operandi for opposing defenses is to push the pocket and the ND offensive scheme/coaches don't seem to do anything to make a defense pay for that strategy.
There are few excellent qbs that don't need an effective scheme or coaches that know how to adapt their schemes to fit a qb's strengths/weaknesses to shine. Unfortunately we have neither which is the key reason the Kelly and this offense has never had a qb who has got better as they got more experience in the system. Simply put, Book doesn't have the size, arm and/or hasn't demonstrated the cleverness/instinct as a passer to make up for the deficiencies of the offensive scheme. He has to get a lot better as a qb just to get to 10-2.