The Midnight Swan — Catherine Fisher

With an invisible girl, a parliament of owls and a pen that writes by itself, the journey to the Garden of the Midnight Swan might be Seren's most dangerous adventure yet. In this third book of the award-winning The Clockwork Crow series, Seren and Tomos must try to help the Crow find the way back to his human form. But why is Captain Jones enquiring about Seren's past? How have the sinister Fair Family gate-crashed the Midsummer Ball, and what is the one desire of the mysterious Midnight Swan?