Fitzhugh wrote two sequels to the book: The Long Secret (1965) and Sport (1979, published posthumously).[24][25] Both books received mixed reviews.[26][27]
Sport is a spin-off that focuses on Simon "Sport" Rocque, expanding upon his brief family background covered in Harriet the Spy. As his parents are divorced, Sport lives with his father, who is a struggling writer who has been focusing on a book (a big gamble) rather than the steady income of journal/newspaper articles, with Sport managing their finances. Their financial problems are exacerbated once Sport's grandfather Simon Vane (from his mother's side) becomes terminally ill and stops sending regular payments to Sport. Things change for the better once Sport's father meets the kind Kate, who becomes a good stepmother. However, Simon's will has named Sport as the main beneficiary to the $30 million family fortune, much to the chagrin of Sport's mother Charlotte Vane and her sister. Charlotte, an absentee mother who has been living well abroad in Europe most of the time, returns to New York City upon hearing of her father's illness, scheming to increase her share of Simon's inheritance by kidnapping Sport and imprisoning him in the Plaza Hotel for a week.[28]
In 2002, a sequel Harriet Spies Again appeared; written by Helen Ericson, it also received mixed reviews.[29][30][31] Another sequel, Harriet the Spy, Double Agent by Maya Gold, was published in 2005;[32][33] one review of that book stated "there's not much to interest readers here."[34]
- Harriet the Spy (Harper & Row, 1964); also Harriet, the Spy
- The Long Secret (Harper & Row, 1965)
- Sport (Dell Publishing/Delacorte Press, 1979), Fitzhugh[35]
- Harriet Spies Again (Dell/Delacorte, 2002), Helen Ericson [and Fitzhugh][36]
- Harriet the Spy, DoubleAgent (Dell/Delacorte, 2005), Maya Gold and Fitzhugh[37]
