His accuracy was a result of short quick read throws. Teams are now challenging that which means tighter windows and initial reads not being open. Teams are blitzing more because he won't take shots down the field and he struggles to see over the middle options.
The answer is running the ball, which they are doing better. Zone Read is embarrassing thought.
Taking more shots, which Book isn't. I do believe those calls and options are there.
Creating an really effective screen game, which is border line criminal.
Do that, ND gets defenses back on their heels and the easy throws open back up.