Helen Ericson, Louise Fitzhugh, . . Delacorte, $15.95 (240pp) ISBN 978-0-385-32786-2

With the approval of Louise Fitzhugh's (author of Harriet the Spy) estate, Ericson revisits the life of Harriet M. Welsch—and the executors' trust was well placed. An author's note reports that Ericson became a fan when this self-styled young spy first appeared in 1964, and her affection for the feisty character comes through in this new misadventure. Even the young detective's fascination with words and her inclination to write her notebook entries in CAPITAL LETTERS endures. When Harriet's parents leave Manhattan to spend three months in Paris, her former nanny, "Ole Golly," returns from Montreal (where she had moved with her new husband) to stay with the soon-to-turn 12-year-old. Though Harriet's mother warns her that Ole Golly has asked that no one mention her husband's name, the curious sleuth sets out to discover what transpired in Montreal. Harriet, while eavesdropping, believes she hears Ole Golly announce that she's innocent, which leads the girl to conclude that the nanny accidentally killed her husband. Meanwhile, another mystery percolates in the townhouse across the street, where husband-and-wife doctors appear to be keeping a girl captive. As Harriet doggedly attempts to crack these cases, her processing of misinformation makes for some comical scenarios. Although the novel does not plunge directly into the mystery (as Fitzhugh's works did) and a few sluggish subplots—including Harriet's creation of a timeline of her life—bog down the pace, overall Ericson has shaped a spirited tale and gives her follow-up to Fitzhugh's novels a fittingly timeless feel. Ages 10-up. (Apr.)