The 3rd book will come next year in 2019 and the 4th is in progress, as Rob Llyod Jones said on his website. The 3rd book might be in Washington, USA, and the 4th could be in Rome.

This was on Rob Lloyd Jones’s website.

This is the last page of the 3rd book.           This is the title of the 4th book.

Seeing these photos, it’s easy to guess the locations. Below is the link to the author’s website.

http://www.roblloydjones.com/news/