Say Boo by Lynda Graham-Barber (2002)

Ben is a little ghost who still has to be able to say “Boo!” on the eve of Halloween. 

He went to a forest, a meadow, and a long stone bridge, to practice to say “Boo!”.  Ben would encounter an owl, a cow, and a dove who told Ben that ghosts do not say “whoo!”, “moo!”, or “coo!” and that the little ghost must say, “boo!”

My son and I have been reading together this book since he was about seven months old.  It’s a small hard book that’s perfect for little hands. 

There's also a nice little lesson at the end of the story: Practice makes the scariest “boo!”