the art of mental training by DC Gonzales
Short super easy read even for a 12 year old.
Mentally Tough Teens was a book I had DD read as a first year 12u. Easy read and helped her with pitching.
While OP's DD is too young to read it (due to length and more in-depth material) .....I'd really encourage the OP to read Mind Gym. It covers positive thinking and how to prepare mentally. There's a lot that applies to everyday life...not just sports. And by reading it there are tools that you can use to help your DD as she grows.