The Princess in Black, Books 1-3

The Princess in Black, Books 1-3: The Princess in Black; The Princess in Black and the Perfect Princess Party; The Princess in Black and the Hungry Bunny Horde by Shannon Hale and Dean Hale

Princess Magnolia is a normal princess. However, when monsters appear she sneaks out to become the masked Princess in Black. Her "unicorn" gets transformed into her black horse. They go and battle monsters, and then return back to the princess duties. The books are all "chapter books", but are very short. The stories themselves are fairly indistinguishable.