Reddit mentions of Jane Savoie's Dressage 101: The Ultimate Source of Dressage Basics in a Language You Can Understand

Most dressage resources talk about both the horse and the rider, since there is really no way to progress as a dressage rider without understanding how your aids affect the horse, nor to train the horse without good riding. So luckily there are a lot of good rider-focused resources out there, but be prepared to read a bit about the horse side of things in any dressage resources you find.

Some that I'd recommend are:

• Lessons with Lendon -- Lendon Gray does a lot of education and has a whole dressage for kids program, so aside from being a great rider she is also pretty tuned in to teaching students across the whole spectrum of experience levels.
  
  
• For the Good of the Rider -- Mary Wanless is a little polarizing, as not everyone loves her brand of biomechanics-oriented riding. But there's no question that she speaks to the rider and that she gets you thinking about what you're feeling as a rider, which is important for developing an innate sense of how to move in the saddle and how your body impacts your horse.
  
  
• Dressage 101 -- Jane Savoie is easy to read and does a good job with the basics. This one is probably targeted most closely to a re-rider getting into dressage.
  
  Your trainer may have other suggestions and will probably be psyched to know that you're excited enough about your riding to do some brushing up outside of lessons, so I'd recommend asking her/him as well.