Book 5. Minor spoilers for earlier books ahead.
In which Harriet collides, full strength, with Cinderella. Her mother holds a costume ball, nominally in honor of the bat ambassador, actually to get Harriet to meet the princes without their past dislike coming into play. It doesn't work, but an astoundingly beautiful hamster shows up.
The rest involves a pet lizard, disease quail are prone to, a stepsister who likes to claim to be wicked, the bat ambassador's sense of humor, and more.