This is an older VHS video, but the info is still great. This video goes over where you should hit your shots when you want to play the percentages. Of course it discusses cross court being a high percentage shot because of the greater length of the court and lower net, but what I liked especially was the discussion of when it makes sense to change the direction of the ball.
According to the video, when the ball is crossing your body on a groundstorke you should send it back in the same direction it came. This works for either forehand or backhand so you don’t have to think too much.
When the ball is not crossing your body is the time when biomechanically and strategically it’s good to change the direction of the ball.
When getting a short ball and approaching the net they recommend hitting the ball 90 degreees to the angle of the net instead of going for the lines.
Lots of other percentage tennis stuff is covered along with drills to make your percentage tennis automatic.
I just checked the Human Kinetics website and couldn’t find this tape there. See if you can find a copy of this video maybe on Ebay (and something to play it on) and your game will improve.