Sometimes I check out a million books at the library and the thing my seven year old reads most is the tattered, dog-eared Lego catalog that his friend passed on to him. Sometimes, I plan out all the good books I want to get at the library and sometimes I walk through the aisles and pull things off the shelves more or less at random (my two year old adding to the randomness and to the amount of books we accidentally check two or three copies of out).
Sometimes the books I think my son will adore are met with stony resistance and absolutely no interest (ahem...Mrs. Frisby and the Rats of Nimh) Other times, books I hated become beloved (A Wrinkle in Time).
Once in a while, I pick out something at random that I've never heard of before and my son carries it off to the bathroom and returns 45 minutes later declaring that it is the "best book he's ever read" and that he's revising his Christmas list to include it as his top gift request.
Such a book for my son is The Cat in Numberland. I grabbed it while walking through the math section at the library and my son fell in love. For days I heard, "Thank you for checking this out. I never understood infinity before. I love this book! Can I read it to you?"
The story takes place in a hotel that is at once always full and always has room for one more. It introduces a lot of mathematical ideas in a way that kids can grasp and makes math (another thing I never liked that my son seems to enjoy) fun.