In Darkling Wood by Emma Carroll

When Alice's brother gets the longed-for chance for a heart transplant, Alice is bundled off to her estranged grandmother. There is nothing good about staying with Nell, except for the beautiful Darkling Wood. But Nell wants to cut it down, Alice feels at home there and even finds a friend, Flo.
But Flo doesn't seem to go to the local school, and nobody else has seemed to of heard of her. When Flo shows Alice the suprizing secrets of Darkling Wood, Alice starts to wonder what is real? Can she find out before the wood is gone for good?

I recommend this book for girls aged 9+.