Monster Handbook by Miss Mary-Kate Martin. Troubles with the Two-Headed Hydra

There are those who hunt monsters to harm them, and there are those who hunt monsters to help them. Who are you?

An ancient mosaic has been discovered on a Greek island, and Mary-Kate is going on a journey with her mother, an archaeologist. While Professor Martin oversees the excavation, Mary-Kate will enjoy sunny days and breathe in the sea air.
But crimes are shaking the island: a boat has been destroyed, a dock has been ruined. Could the legend of the Two-Headed Hydra be more than just a story? Miss Mary-Kate Martin is determined to find the answer!