His turn over ratio is actually rather good, the problem is he plays extremely slow as a point guard, the offense bogs down, he takes too long to bring the ball up court and then he takes too long to engage whatever set they're going to run. It works well for a while but when the game tightens up and the defense locks in, playing that methodical and slow allows the defense to lock in, and create their own mismatches.
For Book to play point he either needs to play more aggressive in the first part of the shot clock or he needs someone really good at the pick and roll to free him up in half court sets.
Regardless, he is like a B- point guard and an elite shooting guard, we really need to focus on letting him play off the ball.