The best way do throw harder and swing harder is to swing and throw!...and properyl! Your technique should come first, as an improper technique can lead to more problems than you might think. Also good technique will help your throw with control and improve contact at the plate.
Anyway, for back: Bent Rows, Seated Rows, Lat Pulldown, Reverse Flys, Good-Morning Bends (which also work the gluteals, your butt muscles), trunk twists while holding a barbell on your shoulders, Chin-Ups, Pull-Ups, and Superman's. But as a pitcher, you also use every muscle in your body, and I think they focus more on their legs and trunk area.
For batspeed:
http://www.batspeed.comMedicine Balls work real well for developing the trunk area. Stand with feet shoulder width apart, and hold a ball about 2-8 pounds in front, arms extended, and twist to the left, then all the way to the right, and back and forth about 10 times or for 10 seconds as fast as you can. Just an example. Baseball players need a full body workout to balance muscle growth, especially since they use every part of their bodies.
"He threw the ball as far from the bat and as close to the plate as possible." Casey Stengel about Satchel Paige
