This is book 13 in the Theo and Ramona series. But can also be read as the first one.
Text on the back cover of the Theo's Pompeii:
One August morning in the year 79 A.D. in the city of Pompeii at the foot of Mount Vesuvius, Theo witnesses something that changes his life forever. As a result, he turns to a fortune teller, and what she tells him shifts him straight into new, frightening adventures in an attempt to evade the disastrous destiny that awaits him.
In this fascinating story of Theo's life in Pompeii, his own story is interwoven with Roman history. The reader follows the drama that unfolds, and that makes Theo the young man he is when he meets Ramona. It is a book full of nerve-tingling adventures in the lead up to the eruption of Vesuvius that destroys Pompeii.
A captivating story governed largely by twists of fate.
Here you can read three chapters of the book Theos Pompeii.