The House at the Edge of Magic 4 : The Lighthouse at the World's End

Nine and her friends are headed to the mortal realm in search of the only thing that will shake a very tricky witch off their tail: a priceless stargold locket. It lies hidden in the Nest of a Thousand Treasures, guarded by Nine’s old foe – the formidable gangmaster of all the thieflings in London. Even if Nine can get her hands on the locket, the friends must find a way to release its power – and for that they must travel through the World Between Worlds to the lighthouse on the Isle of Illusion, where nothing is quite as it seems…