You'll remember Harriet the Spy's friend Sport Rocque as the eleven-year-old who keeps house and account books for his impractical writer father.
This takes place before Sport turns twelve (presumably then still in the Sixties, though there's an anachronistic reference to the hustle), during the weeks that Sport starts in at a new junior high school, his father marries a super, sympathetic lady named Kate, and his mother's father dies, leaving most of his immense fortune to Sport. But as Sport's mother's share of the inheritance will double if she stays home from her European jaunts to raise him, she proceeds to kidnap her unwilling son, locking him up first in his grandfather's home and later in the Plaza Hotel, from which Sport escapes in a food cart propelled by his friend Chi-Chi who's a bellhop there. (How he got the job at his age isn't considered.) The fast-paced story has its share of funny scenes, and fans of Harriet will be interested in Sport's own story—but it's a vastly inferior one, with nothing more on its mind than Sport's escape from his witchlike mother.
And—where the characters in the first book were memorable, larger-than-life caricatures—Sport's mother here is merely a predictable and uninteresting stereotype; Kate is too perfect an answer to the Rocques' prayers; and Sport's three friends—one Jewish, one black, and one Hispanic—seem an unrealistic vestige of naive Sixties didacticism.