Probably Pistachio is one of Stuart J. Murphy's Math Start books -- quite possibly my favorite series of math books!
The concept for this book is probability. As Jack goes through his day he is always thinking about the probability of certain things happening. What makes it fun is that everything is going wrong for him on this particular day. In fact, the book very much reminded me Alexander and the Terrible, Horrible, No Good, Very Bad Day! You can use this book to talk about math vocabulary: probably, impossible, and certain. You can have students make predictions and talk about fractions. Follow up with your favorite probability activities (colors in a bag for example).