Times Through the Order
Hitters get better each time they see the same pitcher in a game.
How it actually works. The penalty is real and it is large: roughly the difference between an average hitter and a good one by the third time through, and it holds after controlling for pitcher quality and fatigue.
Why it is done, and when it is wrong. This one idea explains most of how modern pitching staffs are built. It is why starters go five innings instead of eight, why the opener exists, and why a manager pulls a pitcher who looks fine. The hitter has seen the pitches, and seeing them is most of hitting them.
Batter's Eye: look up anything in baseball