Narrator O'Diddy is an imaginary friend, named for the classic parental response to a child who attributes a prank to one of his kind: ""Oh, did he?"" He is here assigned to Boon, whom he finds ""rare and special."" The two have several happy years together before Boon starts school, makes real friends, and forgets O'Diddy, making him invisible--and also inaudible. Still, on one memorable occasion, she remembers him long enough to allow him to help her escape a peculiarly embarrassing situation. O'Diddy is garrulous, and his pleas to the reader to stay with the story, and with him, grow a bit tedious--though they are understandable, given his situation. But having an imaginary friend as narrator is a novel notion, developed with ingenuity and some humor--the latter considerably augmented by Truesdell's comic, perceptive drawings. Promising fare for young readers.