The Princess in Black Takes a Holiday by Shannon Hale

After battling monsters all night, a sleepy Princess in Black decides that she needs a holiday. After all, the Goat Avenger has offered to protect the goats while she’s away. The very next day Princess Magnolia rides her bicycle to the seaside, where the air is salty, the sun is shiny, and the sea is as blue as monster fur. But just as Princess Magnolia is about to take a nap on her hammock, she hears a ‘ROAR!’ Could a sea monster ruin this holiday for the Princess in Black?