My Childhood Hero: Johnny Ball, Think of a Number. London: Dorling Kindersley, 2005.
Utterly brilliant. Each page has clear diagrammes, and consists of information, followed by examples, followed by problems. I can't do justice to this book. Just go buy it. If your kids don't love it, you will.
One of the best pages is the one in which a world without numbers is imagined through news headlines: a lottery of coloured balls; a game with "lots and lots of goals"; a high jump record 'a bit higher than the last record".
There are sections on the history of numbers and maths, there is instruction on geometry, and excellent pages on patterns in numbers such as the Fibbonaci sequence and Pascal's triangle. And Johnny Ball becomes my hero for explaining why all of this is so important.
Crucially, the book is information dense. It expects children to get excited and it deliberately provides more than they could possibly manage (excepting the odd genius) on each page.
I'm resitting my maths GCSE this year and working through this book slowly ths summer is going to help enormously.