Tim Gallwey was ahead of his time with this book (circa 1972?) My dad was a tennis player and suggeste dthat it could help my hitting when I was a boy as there wasn't much about thi stopic from the baseball side.
Ken Ravizza and Harvey Dorfman come to mind if you are looking for baseball specific applications. Also the Science of Hitting has some very good stuff on Mental aspects although it is not as "scientific" as some of the more recent publications. You can see why Ted Williams was a great hitter!
Some books I have use din my instruction and that I feel are beneficial to the mental side! All are available from Amazon.
The Mental Game of Baseball: A Guide to Peak Performance Karl Kuehl (Author), Harvey A. Dorfman (Author)
The Mental Keys to Hitting: A Handbook of Strategies for Performance Enhancement Harvey A. Dorfman (Author)
Coaching the Mental Game: Leadership Philosophies and Strategies for Peak Performance in Sports and Everyday Life [Paperback]
Harvey A. Dorfman (Author)
The Mental ABC's of Pitching: A Handbook for Performance Enhancement H. A. Dorman (Author)
Science of Hitting Science of Hitting
Ted Williams (Author)
John Underwood (Author)
Heads-Up Baseball : Playing the Game One Pitch at a Time Tom Hanson (Author)
Ken Ravizza (Author)
Mental Toughness: Baseball's Winning Edge Karl Kuehl (Author), John Kuehl (Author), Casey Tefertiller (Author)