Reddit mentions of Lessons with Lendon: 25 Progressive Dressage Lessons Take You from Basic Whoa and Go to Your First Competition (Popular Training Series from Practical Horseman)

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.

Wow, that genetic test yielded such interesting results! There was actually a fascinating discovery a few years ago that found a link between a specific gene mutation and the ability for a horse to have extra gaits, like the pace. Here's a cool article about it I don't think it directly applies to your guy but there is definitely a genetic link to the way that horses move.

I think one very good intro to basic dressage book would be Lessons with Lendon, and luckily it's available for $3.99 on Amazon! It starts with the real foundations of dressage and is written in clear language with very good pictures and easy to follow captions. I think it would be helpful for anyone interested in dipping their foot in the dressage waters. My next favorite book is Dressage Formula by Erik Herbermann but it's a little more dense and theoretical. If you find that you really take to dressage and would like to delve deeper, that's where I'd point you. My favorite thing about that book is that the author illustrates his principles on off-breed horses, like a Standardbred and a big clunky part-draft foxhunter. They look freaking amazing in the book and it's a good demonstration of how the basic dressage principles are meant to improve every horse's way of going and balance.

Good luck with everything! It sounds like you're off to a great start though! I'm glad to hear your guy hasn't been cross cantering. Cantering can be tough work! If you have any more questions, I'd be happy to help as best as I can!