Handbooks need to layoff, sometimes.

I am familiar with the term. It is a 'retail' bookmaking operation - the pool hall owner would run the bets through a bookie syndicate and get a small cut.

Bookmaking on horses, baseball, football, whatever.
Probably not big time, just for regular customers who want some action. The bookie might take 10% off the top. He would also need a place to lay off bets that are out of balance, so if everybody bets on say, San Francisco to win and it does, he has a larger bookmaker to bail him out.
The pool hall might just break even, but the book would make it profitable.

They had to be talking about running a pool hall and doing a little bookmaking on the side. It's the only thing that makes any sense. Small independent bookies were said to be running "a book". I'm guessing "handbook" was another term for small time bookies in the 50s when the novel was written.
People could play some pool and put down bets on whatever was worth betting on and the house got a cut. To think a "handbook" was some kind of guide on how to play pool is just silly. Any pool player who needs a handbook to tell him to play has no business being there.
God's Holy Trousers!!

It makes sense that a pool hall would have something like that to give rulings on the rules of the games. I'm sure that Charlie would have one of these when he opened his place, but in regards to the movie there is no doubt in my mind that "handbook" referred to taking bets.
from Merriam-Webster.com (definition of handbook):
2. a : a bookmaker's book of bets
b : a place where bookmaking is carried on

Or maybe he was talking about this handbook?
http://www.deceptology.com/2011/05/handbook-for-pool-hustlers.html

Can't he just buy a 'handbook' right now? I still think this references bookmaking.

Handbook on the side is the rules on all the different pool Games.
All the old pool halls I'very been in have the handbook to settle disputes over the rules.

I assumed it was bookmaking. Perfect place for it don't you think.