The best book I have seen is Cheryl Kemps book published by Human Kinetics lab.
I have rarely heard any off the experts argue too much with what she lays out in the book.
That said, a good instructor will make all the words in the book actually make some sense and it may be that the congruence of my daughters instructor, and 6 years of lessons, and the book made it all at least reasonably clear....
i've it all books and dvds of the world including Cheri's book,that is quite good but...Barry Sammons-The windmill pitcher it's really the best.You could find there any aspect of the art of pitching including difference between all styles and timing motion.Great help to me was read Eddie Feigner's "Little i know about pitching and hitting",little book with wonderful pictures about good old days of fastpitch and full of tips and tricks about your health,what to do and what not,and when and why...a little treasure.Talking about dvds Maybe Ernie Parker's series it's more easy for a little girl,instead to me greatest it's Bill Hillhouse's "Building the house" and "the road to the rise".All in all i think that greatest tool it doesn't exist,it all depends if who need to learn it's open to do it.In that case anything could help including wrong advise because you could see by yourself that is bad for you.Cannonball it's also a great tool but you need to use it very careful and weighted ball and light ball helps.
My daughter has been working with a pitching coach for the past three years and one of the tools he recommended was the "Xcelerator" it's a soft canister attached to a softball by a rope. The pitcher holds the softball normally then twirls the canister in the windmill motion. It's great for warm ups, muscle memory and mechanics. You or your daughters' coach can watch her arm motion and see if see is using the proper rotational plane. You can quickly simulate 50 - 100 pitches without the stress of ball release. It only costs $35 and is available at www.softballoregon.com. My daughter also uses a canonball 1 lb. weighted ball $20 and we've recently acquired a Finch Windmill $350. I've got a Tight Spin Trainer $40 on order as well.
I started out with the low dollar equipment and have gone to the more expensive as she has progressed. The key is determining her level of interest and what she needs to continue to progress.