Lady Friday by Garth Nix

In this thrilling installment of the Keys to the Kingdom series, Arthur Penhaligon finds himself facing the unpredictable and enigmatic Lady Friday, who has abandoned her duties as the Trustee of the Fifth Key. As Arthur navigates the treacherous challenges of the Middle House, he must contend with Lady Friday's insidious plans to harvest the dreams of mortals. With the help of his loyal friends and allies, Arthur races against time to prevent chaos from spreading across the realms, all while grappling with the growing burden of his own responsibilities and the looming threat of the mysterious Nothing.

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