To what person does the title of Book the Second, "The Golden Thread," refer? Why is this title a good one?

Lucie is the golden thread, as she holds all the other characters together. Lucie is the one thread of her father's past that remains, and she is the person who provided him with love and support. As a character, we don't see Lucie as having much life or flamboyance..... but it is her quiet demeanor and "golden" or sunny dispostion that binds everyone together.