Googlebooks review as my review lost by LibraryThing App:-

"This gripping, page-turning fantasy adventure follows a dangerous quest through a divided world.

From the high reaches of a tree, Fliss watches the soldiers attempting yet again to break through the invisible wall. Amid the explosions, a drummer boy tries to escape. As he is about to be shot, Fliss reaches through the wall and pulls him to safety. But Fliss is dismayed to find she has saved an overfed rich boy. She is even more dismayed to learn that she must accompany him back through the wall on a special mission to rescue the Nightingale.
The world they have to travel through is a perilous one, full of predatory thieves, slave masters, beggars, dippers, mudlarks, drain-sliders, spies and wall-men. It is a world where the ruling families are caught up in a lethal power struggle.
Will Fliss and the despised drummer boy learn to trust each other? Who is the Nightingale? And will they all make it back alive? " ( )

The setting for this book is novel. An invisible wall has been created by the strength of joined minds. The northern side lives peacefully while the southern side is full of disharmony. When young Fliss saves a drummer boy, Kirt, from death by pulling him through the wall she sets in motion a chain of events. The 'Old One' asks to speak to her. his is the only mind now holding the wall and he is dying. He has located another mind capable of taking it over, Kirt's blind and disabled sister, Laura who is entrapped in the southern land. Their mission is to rescue her and bring her to the northern land to take over holding the wall before the 'Old One' dies. an adventure ensues...
I was curious to read this novel as I am a fan of his adult fiction. It was an undemanding and some what predictable read, designed for a young adult audience. ( )