Getting the right answers in maths is only half the problem. Understanding why what you're doing works is the part that often stumps students and teachers alike. Does maths feels like a collection of random rules and steps that somehow lead you to an answer? Don't worry you're not alone.
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.
Maths 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.