These books blurred together. It is less than an hour at slow speed for all three together. The one that stood out was the one where two princesses adopt secret identities to fight monsters, without letting each other know of their alter egos. There is also a monster that serves as the science fair project. Not quite sure what the right audience is for these.
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.
