The Strange Disappearance of Imogen Good, by Kirsty Applebaum (paperback)
 
A hidden garden. An ancient tale. A missing child. This is an addictive mystery with crossing narratives that's impossible to put down.
 
Fran doesn't want to stay with her aunt and uncle and her annoying cousin, Imogen. Imogen is rude and unfriendly and, it turns out, missing... But her parents don't seem to know who Fran means - don't be silly, we don't have a daughter, you know that. So it's up to Fran, and Imogen's best friend, Bex, to find out what's happened to her.
 
Except Bex doesn't know who Imogen is either. All Fran knows is that it's got something to do with the hidden garden at Stillness Hall and the twelve statues that belong there. Could a mysterious old tale, a story of enchantment and death, hold the answers?