The Princess in Black and the Perfect Princess Party by Shannon Hale

Today is Princess Magnolia’s birthday party, and everything is going wrong. Just as her guests are arriving – Brring! Brring! It’s the monster alarm! Princess Magnolia has to run to the broom cupboard, ditch her frilly clothes, and become the Princess in Black! She rushes to the goat pasture, defeats the monster, and returns to the castle before her guests discover her secret. But every time Princess Magnolia is about to open her presents, the monster alarm rings again. And every time she rushes back – an inside-out dress here, a missing shoe there – it gets harder to keep the other princesses from being suspicious. Don’t those monsters understand that now is not a good time for an attack?