"Return of the Ghost", McCallen B.

Abby and her friends had a unique opportunity to take part in the tournament of the most talented and gifted children of America. And this means that they will not only go to the Cool School of the BRIR and try to fight for the title of the most intelligent teenagers of the country, but will also be able to prove that excellent spies will come out of them. But in Briara, in addition to special tasks for ingenuity and knowledge, many dangers await them. A ghost, a powerful criminal, is hunting for the treasure, which is owned by students of the Smith's school. But neither Abby nor her team suspect that they have something that may interest one of the main villains of the world.

The third book of the series.