What is your favorite "children's" book for adult life lessons? Why?

What "children's" book have you discovered to have some critical messages that apply to adults? I collect these sorts of books because I can use them to teach children - as well as adults from time to time, about important character traits that I would hope we as human beings would exhibit. My favorite of all is Dr. Seuss' "Hooray for Diffendoofer Day." Why? because it screams that EDUCATION and teaching people to THINK is more important that indoctrination and simply teaching people to pass tests. I love this message. What is your favorite such book and why?