The Princess in Black and the Perfect Princess Party (The Princess in Black #2) by Shannon Hale & Dean Hale

WHO SAYS PRINCESSES DON'T WEAR BLACK?

Will Princess Magnolia's party guests discover that she's secretly . . .
The Princess in BLACK?

Today is Princess Magnolia's birthday party, and she wants everything to be perfect. But just as her guests are arriving . . . Brrring! Brrring! The monster alarm!
   Princess Magnolia runs to the broom closet, changes her clothes, and becomes 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. Don't those monsters understand that now is not a good time for an attack?

(Illustrated by LeUyen Pham)