I think F&H is actually bit easier to accurately summarize, just because all the real mindfuckery comes at the end - up til that point you're pretty safe with "Polly has just realized she has two sets of memories, and one of them involves a surreal friendship from her childhood with a man who may be under a curse." Whereas with Hexwood you just have to start off with "so thirteen-year-old Ann is home sick from school, then an amnesiac assassin made a kid out of her blood and then there were dragons and robots and evil reigners and a castle full of bandits and the wood is manipulating time so everything happens twice and, wait, where are you going . . ."