The Traitor's Blade Volume 5 (Blackthorn Key) by Anne Ursu: Christopher's homecoming takes a sinister turn when a murderous conspiracy is uncovered in this fifth novel of the award-winning Blackthorn Key series.
Christopher, Tom, and Sally are back in London at last. Everyone is relieved at their return--the plague has ended, and the king, pleased with their service, offers a surprising reward. But trouble has followed them home. . . First, an old friend is ambushed and left for dead. Then an anonymous letter arrives at Blackthorn--a mysterious warning hidden inside a riddle and secret code.
As Christopher and his friends begin to investigate, they soon discover they've stumbled upon a plot to kill the king. . . and anyone else who dares stand in the traitors' way.
Perfect for 8 - 12 years readers and fans of fantasy.