These Dark Wings

After her mother is killed in the Blitz and her father in the North Sea, 12-year-old Anna Cooper is sent to live with an uncle she has never met – the Ravenmaster at the Tower of London.
Amid the Tower's old secrets and hidden ghosts, the ravens begin to disappear and Anna must brave the war-torn city to find them.
With Nazi forces massing on the other side of the Channel, the fate of Britain might be at stake, for an ancient legend foretells that Britain will fall if the ravens ever leave the Tower.