I'm one of those rare old schoolers who loves watching great D. I get just as much a rise out of a great Defensive play than an Offensive play. So I do tend to focus more on Defensive ability when I watch players. (Ie, I will forever argue that Klay was the 2nd best player on the floor with Durant night in and night out.)
I've been watching Book also for these past few years. What I've seen is easily one of the worst Defenders in the league. I don't mean just SG, where he was the worst, I mean just overall one of the worst defenders in the NBA. I don't base that off of reputation, but rather what I observed. He was slow footed, slow to rotate, showed poor footwork, reaching instead of moving his feet, poor communication. and displayed OLAY D. He would have bursts of energy on D when it mattered (which was hardly ever because we sucked) only to see him give up 10-15 seconds into the possession.
What I see this year.... Um... I'm totally stunned. He's exhibits far better body position, plays through the possession, communicates with passion, rotates with great energy, keeps his feet moving, and maintains decent discipline with his hands. I'm really impressed that he holds his defensive body position throughout as I've NEVER seen him do that before. Defensive is ALWAYS played with heart, something he rarely showed in the past on D. Now, ALL I see from him is heart. He is hitting a whole other level. It reminds me a ton of what Steph Curry did in GS. He isn't the most talented defender on the court but he goes all out for his team.
One of Monty's many outstanding accomplishments this year is getting this kind of effort on D from Book... and Kelly... and Frank... and Dario... and Jerome... and Ellie... and HOPEFULLY Ayton.