If you don't like Harrow, I wouldn't continue the series. Nona the Ninth is more of an interlude (personally it's my favorite of the three), and sets up more questions than it answers. Unless you really loved Camilla Hect and Palamedes Sexton (because Nona is their book along with Nona herself). Also, the final book, Alecto the Ninth, still seems to be nowhere in sight (on goodreads there's no release date set). It's been years now.