Although the novel certainly succeeds as a thriller, building suspense after suspense around—but not limited to—the disappearance of Heather Kelly, it is really more about growing up. As the older of the two sisters, Clare Kelly, the "sensible" one, takes responsibility for not only her younger sibling, but for her mother, too.

Throughout the novel she is deeply concerned for Heather's health and mental stability, and she is right to be, as things turn out. But she also has to Has Anyone Seen Heather?

hold the rein on Katie's impulsiveness, which gets them into difficulties, sometimes humorous ones, but more often serious ones, as when they open the door on an attempted rape in the hotel where they first work, or later when they sneak into Heather's employer's house to try to retrieve their mother's belongings. All of Clare's experiences...