Kids fantasy. Book found in attic that releases mythical creatures

[Maggie]

The main protagonist is a young boy. He gets bored and finds a pull string ladder to the attic where he finds a magical book that releases mythical creatures. He and his friends have to return all creatures before the week ends? Or the full moon? There is a unicorn in it.
Younger brother read it from school library around 2012 when in fourth grade

[Becca]

This sounds like The Revenge of the Shadow King by Derek Benz and J.S. Lewis.