Recently read: Introductory Calculus For Infants
I'm not really sure how to describe this book: If you expect a calculus book, it has less calculus than you might expect. If you're expecting a kid's book, it has more calculus than you'd expect.
It follows the letter x (which the other letters didn't like) after f (who is fabulous and fun) explains how as a team they can do lots of fun things: one for each letter of the alphabet, all at least plausibly calculus-related. There's a happy ending: The other letters see how x can be a good friend.
I look forward to reading it to Peter and Amber, though I expect that I will have a lot of explaining to do. (Despite the title, I don't think I would try reading this book to a very young child; an appreciation for the alphabet is necessary.)
Conditionally recommended: It's cute and funny, but would presumably not shine as well if you're not in the mood for a kid's book, or don't have fond memories of calculus class.