I actually have so many that I can’t quite chose one haha depends a lot! But if you’re interested in the science part of learning, start by the behaviorism ones! Skinner & Keller specially. For training, Karen Pryor’s don’t shoot the dog is a classic and I think there is a new version. On neuroscience, Jaak Panksepp is our most influential author.
If you want anything more specific feel free to dm or ask more! That’s my favorite topic hehe