1. Mental Maths P1 - to develop mental math skills
2. Heuristic and Model Approach P1 - to develop process skills
3. A topic by topic assessment book e.g. Fan Learning/Andrew Er.
4. Use manipulatives to teach. You can buy from PLAY N LEARN
These are the minimum I would invest on. As children have workbooks, worksheets, etc, I pick books based on complementary material that my nephew needs so as not to overwhelm the child.
I am not a staff of any of the above companies that sell these titles. I sell Maths books to the export market and wrote an ebook on the strength of our Maths program and how I taught my nephew since he was a toddler, what books to use, research about Maths learning and how I applied it to teaching.