It helps to have a grasp on the mental aspects of hitting, too.
To develop a good swing routine and add more focus to each at-bat, we recommend the training DVD by Tom Hanson called "Zoned In". It will show you how to put measurement to mental training and how to develop consistently powerful at-bats. Click here.
To understand how pitchers are learning to deceive hitters with "downright filthy pitching", you can benefit from the work being done by Perry Husband. His latest books are directed at pitchers, but every hitter needs to be taught what the "enemy" is up to. Click here.