The third book, *The Grey King*, was published in 2010. While the series has a dedicated fanbase and received critical acclaim, there hasn't been any confirmation from Nancy Farmer or her publishers about future installments.
However, it's important to note that:
* Authors can change their minds: While no news is out now, there's always a possibility that Farmer might decide to revisit the world of the Sea of Trolls.
* Fans can voice their desire: If you'd like to see a fourth book, you can reach out to the author or her publisher expressing your interest and support.
While we wait for any official announcements, we can keep our fingers crossed and hope for a return to the exciting world of the Sea of Trolls!