The Princess in Black Takes a Vacation by Shannon Hale

Even monster-battling princesses get tired sometimes! But a peaceful time away is hard to find in the fourth book of the New York Times best-selling series.

After battling monsters all night, a sleepy Princess in Black decides that she needs a vacation. 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 vacation for the Princess in Black?