Axiomatic set theory books and foundation of math through logic books are mostly difficult symbolic jungles that leave the reader without inspiration. Many readers will come from these books feeling that understanding the structure of elementary math is beyond them.
The Dedekind 1888 book, “What are and what should be the numbers?” however is within the reach of most teachers and when mastered results in a visionary understanding of the structure of sets, counting, functions, and addition and multiplication.
This is the royal road to the structural math of K-6 math. No one should shirk learning this book well. It does require effort. This webpage and my books on Peano Axioms will make it much easier to get up the learning curve. My materials contain many elementary examples and exercises in contrast with the existing materials. This gives teachers a stair case instead of the rock face that conventional materials offer them.