Ask yourself: why do we have odd and even numbers? Why do two negative numbers multiply to make a positive? Why do fraction operations work? What is cosine and where does it come from? Yes, but why? answers all of your questions, and sheds light on the hidden connections between everything in mathematics at school. Math makes sense. It always has, but until now maybe no-one ever showed you.
A must-read for those training to teach primary or secondary mathematics via university-based (PGCE, BEd, BA w/QTS) or school-based (School Direct, SCITT, Teach First) routes and current teachers wishing to deepen their mathematical understanding.