The Code of Romulus by Caroline Lawrence

A short mystery from Caroline Lawrence's Roman Mysteries series, set in the port town of Ostia. Determined to prove she's a true detective to her tutor Aristo, young Flavia Gemina takes on what looks like a simple case: someone has been stealing bread rolls from the local baker. But the trail leads her into a puzzle of wordplay and hidden meanings involving a slave named Romulus, turning a minor theft into a clever exercise in logic and codes. Published in 2007 to mark the 10th anniversary of World Book Day, this slim, fast-paced story sits between "The Dolphins of Laurentum" and "The Twelve Tasks of Flavia Gemina" in the series, and works well as a light, self-contained introduction to Flavia and her friends.