Sir Hereward And Mister Fitz by Garth Nix

An aging, duty-bound knight and his quiet, enigmatic companion are pulled into a series of strange, dangerous encounters in a decaying, magic-tinged world; their journey mixes adventure and investigation as they confront both supernatural threats and human corruption, revealing the costs of loyalty, the ache of growing old, and a wry, sometimes dark sense of humor.

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