Anna Nilsen has hit on a novel idea with her two books of flip-cards. They consist of three separate packs of cards in ring-binders inside a stiff cardboard wallet, with sums on them of increasing difficulty. Using pictorial objects to help you count, you work out the answer and then flip the card over to see if you've got it right.
The books are aimed at children aged four plus, who are just starting to count and add; but older children, for whom the sums are easy, will have great fun flipping the cards back and forth. The idea, according to the notes inside, is to introduce young children to the concepts of adding and sutracting while allowing them to progress through the cards at their own pace. My reservation here is that, for a child not yet confident with the concepts, the books could feel a little too like a workbook to be ploughed through, especially if a parent is beside them driving them on.
Also, unlike the maths-through-stories approach fast gaining popularity, where children have a real-life context and a purpose for their mathematics, Anna Nilsen's books fail to give children any real reason for doing the sums - other than simply to get the answers right.
One design cavil, too: spiders, insects and squirrels are fine for counting, but the coloured-in circles on some of the cards looked to me confusingly like noughts.