The Mask Falling by Samantha Shannon

The wreath of flowers that Paige finds in the tunnels under Paris (Chapter 24) – a message from Jaxon:

White puffs of bittercress: “paternal error” / Monkshood, purple and poisonous: “treachery” / Fragrant bay leaves with golden blooms: “I change but in death” / Barberry, like drops of blood: “sourness of temper” / White clover: “think of me”