This is Book 3 in the Century series. Book 4 has been written in Italian but doesn't appear to have been translated into English as yet.
The adventure continues with our 4 teen-agers who must save the world without knowing that is what they are doing. The enemies continue to pile up but surprising friends appear and one enemy turns friend. The children begin to confide in their parents' in order to get their help as well as continue to enjoy the freedoms needed to complete their tasks.
I am enjoying this series in part because each book is set in a different city (this time Paris) and a great deal of ancient history is incorporated.