I started Writing Picture Books by Ann Whitford Paul. The author provides an exercise at the end of each chapter so you can apply what you learned. The exercise not only educates you in your writing, but might help you further your PB ms. I changed one of my PB based on the results of an exercise. I hope you find this helpful too. I bought it used inexpensively at half.com. In fact, I got several books on writing through that website at a great price. Used, but in great shape.