This book is a fitting sequel to Harriet the Spy; it retains some of the same characters and adds fantastic new ones, and we get to see some more depth from some of the side characters, if not so much from Harriet herself. I like how the author develops the titular "secret" and how she just drops us into the action and lets us observe how things unfold. I also like that not everything is neatly resolved; while we do discover the secret and etc., there are a lot of things that we don't find out about, which is realistic.
I feel like there could be a lot more stories that take place with this crew, but sadly there aren't. There is a book that is supposed to be about Sport, but I read it years ago and it had none of the magic of this book and the predecessor; I think it was published posthumously so it might have been a rough draft or have been partially written by someone else or something. Like Shirley Jackson, Fitzhugh died young and it makes me sad to think of all the great books she might have produced. Another childhood fave that I can hold close to my heart.