There must be a pony!

I want the book that this postcard is of!
It reads: Once-upon-a-time a psychiatrist had twin sons age eight. One was an incurable pessimist - the other an incurable optimist. Their father became alarmed and decided to try an experiment. Christmas Eve he filled the pessimists room with everything a boy could wish for; and the optimists room with horse manure. Early next morning he went to observe their reactions. The pessimist sat among the toys, books, clothes, sporting goods - just sat there - eyeing the presents suspiciously, trying to figure out what the catch was. His father sighed and walked towards the other boy's room. When he peeked in the door he saw him standing waist high in the middle of all the manure, shovelling it up in the air over his shoulder and laughing like a fiend! 'Son,' the father said, 'what's the matter with you? What are you so happy about?' The boy turned, still laughing, and replied, 'Gee, Dad, I figure with all this horse shit - there must be a pony!'
I'm not laughing right now, but I'm darned well still looking for the pony! And I realised that it's partly a case of believing in the pony, and also a case of going out and looking for it. That's me.