If looking for a unique series of math instruction books,
Life of Fred may be of interest.
Each of the children's books is named alphabetically (Apples, Butterflies, Cats, Dogs, etc). College level books are also available in the series. Each lesson is built around a simple story in the life of
cartoon character Fred Gauss, and provides the
reason why such a calculation would come in handy.
While some kids love this approach, others do not enjoy it. The author is a Christian pastor, some people do not consider the books for that reason.