The Roman mysteries omnibus - Caroline Lawrence

It is AD 79. Join Flavia Gemina, a Roman sea captain's daughter, Jonathan, her Jewish neighbour, Nubia, the African slave-girl, and Lupus, the mute beggar boy, on three exciting adventures.
In THE THIEVES OF OSTIA, the four friends get on the trail of the killer who is silencing the town's watchdogs. Not long after solving this mystery, THE SECRETS OF VESUVIUS finds them relaxing near Pompeii. When Mount Vesuvius erupts, they flee one of the greatest natural disasters of all time. Sheltering in the refugee camps following the eruption, they discover that children are being kidnapped from the camps in THE PIRATES OF POMPEII. Determined to find out who is responsible, the friends come face to face with pirates and slave-dealers . . .
Three captivating mysteries: compulsive reading for 9-11 year-olds.