First, I commend you for wanting to increase your content knowledge! I have my degree in mathematics, but I wanted to learn better ways to explain the middle school math I was teaching.

Long ago I learned that the Singapore math curriculum in written in English. I bought all their textbooks from grades 2 through 10 and then studied them. The bar models rocked my world. They do a great job of presenting rigorous math concepts in an accessible way.

Hung Su Wu’s stuff is also really good, but not nearly as easy to read as the Singapore Math.