The Sisters Grimm Book 4: Once Upon a Crime (Sisters Grimm)by Michael Buckley
This was my favorite book in the series so far. I thought it was really funny. The only thing that could be considered questionable is that the Emperor from Emperors New clothes makes a brief appearance.
To save Puck, the Sisters Grimm must go to Faerie (a world hidden within New York City). While in New York Sabrina learns a shocking secret about her mother, and decides to give up the family business. But when Puck's father is murdered Sabrina agrees to solve one last crime before she retires for good.