Reddit reviews Elementary and Middle School Mathematics: Teaching Developmentally (8th Edition) (Teaching Student-Centered Mathematics Series)

If you want to understand the conceptual underpinnings of elementary mathematics, the very best book for you to learn about K-8 mathematics concepts is John Van de Walle and colleagues' Elementary and Middle School Mathematics: Teaching Developmentally.
While not exactly a textbook, there are narrative parts and chances for you to "try it out". I would recommend this book over actual textbooks (although Investigations is the best K-5 textbook out there), because primary grade textbooks would be too simplistic for you.
Also, while this is an expensive book to buy new, there are older editions that are much more affordable and almost all colleges and universities that have teacher education programs own at least a copy or two to borrow.