If I'm betting on the game, my #1 rule is bet against a team that has one of the base coaches keeping book.
You must know a lot of coaches with a sub-85 IQ who can't do a couple very simple things at once.
I've never one time had an issue keeping the book and coaching a base. Even managed to do it with a broken hand, once I was able to hold a pencil again. I'm certain there has never been a time while coaching a base that I somehow, I don't know, missed a play or something because I was writing in the book. I don't even know how that could happen. There's like 20 seconds of down time between every pitch.
It can be slightly more difficult to do the book and call pitches (sometimes I call them; sometimes the catchers call them), but it's still easier than asking someone else to keep the book and just asking them questions every 20 seconds.