I think Book is doing an effective job with what he has. He just doesn't have arm strength which is exhibited in more than just downfield passing. For example, the quick passes to the outside wrs take a long time to get there allowing dbs to close and shut down the play. There was another downfield play, don't recall whether it as Claypool or Kmet, where the ball was on target but it took a long time to get there allowing the db to close down and breakup the pass. The problem wasn't recognition and timing as much it was just it took too long. This is even more obvious in person when you see a rather pedestrian USC qb get it there on similar plays so much faster due to superior arm strength.
It is always going to be his, and the offense's, limiting factor. In a lot of ways, Book is this year's Joe Schmidt: effective at his job but not going to scare anyone which hinders top level play.