The following paragraph is from Don Winslow's novel The Force:
Malone and Russo and Billy O and Big Monty and the rest made these
streets their own, and they ruled them like kings. They made them safe
and kept them safe for the decent people trying to make lives there,
and that was their job and their passion and their love, and if that
meant they worked the corners of the plate and put a little something
extra on the ball now and then, that’s what they did.
I can guess that "worked the corners of the plate" and "put a little something extra on the ball" are baseball idioms and these two probably mean doing something unscrupulous or cheating. But I don't watch baseball and I have a hard time understanding why and how "working the corners of the plate" would indicate dishonesty.
Best Answer
As you assumed, both are baseball references. In baseball home plate horizontally defines the strike zone. (The batter's knees and shoulders define it vertically.) When a pitcher is working the corners of the plate they are only barely throwing strikes--a pitch outside the corners would be a ball.
It is of course a metaphor to make throwing a ball the equivalent of unscrupulous behaviour, but it makes clear enough sense. Both mean to act outside of how you're supposed to.
To put something extra on the ball refers to spitting on the ball or tampering with it in any other way, which is illegal as it makes the ball move in unfairly unpredictable ways.