Advanced Techniques of Riding-German FEI for a basic.
The Classical Seat by Sylvia Loch- for what and why and how your seat should work
Riding Logic-by Muesler, but be prepared for him.
Podhajsky is also great-The Complete Training of Horse and Rider.
If you're are advanced enough -The Athletic Development of the Horse-DeKunffy
Some riders change their horse, they change their saddle, they change their teacher; they never change themselves.