Becoming the Math Teacher you Wish You'd Had by Tracy Zager and Big Ideas from Dr. Small, were the first two resources I was introduced to that helped me begin growing as a mathematician. "Creating a comfort zone for teaching mathematics" was what I needed to push myself and my students forward.
Graham Fletcher has created animated progression videos of addition, subtraction, multiplication & Division. I find these helpful in understanding where students are at, where they came from and where they are going. Please keep in mind that these videos are based on American curriculum.