The Roman Mysteries Omnibus by Caroline Lawrence
THE THIEVES OF OSTIA: The theft of her father's signet ring leads Flavia Gemina to three extraordinary people - Jonathan the Jewish boy, Nubia the African slave-girl, and Lupus, the mute beggar boy - who become her friends, and partners in solving the first of many mysteries. Will they discover who is killing the dogs of Ostia, and why? THE SECRETS OF VESUVIUS: Flavia is spending the summer with her uncle who lives near Pompeii. She and her friends are absorbed in trying to solve a riddle that may lead to a great treasure. Then Mount Vesuvius erupts - and the four children find themselves in desperate danger. THE PIRATES OF POMPEII: Following the volcano's eruption, hundreds of refugees from the cities of Vesuvius try to come to terms with disaster. When children begin to go missing from the camp, Flavia Gemina and her friends investigate a powerful and charismatic man known as the Patron - and come face to face with pirates, slave-dealers ...and death.