On rewatch, I wonder if he was on the way to suggesting someone like Book when he was interrupted
Agree with this, it seemed like he was using Book for context, not for a real suggestion. Saru respects what Booker has done for everyone, but I don't think he's a real big fan of the guy, and certainly doesn't think he should be the XO of a Federation ship.
I could be wrong, though
I couldn't locate this specific one on Amazon or Paramount, but fortunately i had a local copy :D