This volume: The courtship and early marriage of William's parents. Emma returns to Haworth changed. Eleanor's father appears and is pretty freaking scary.
I love the story of William's parents, which explains a lot of reactions and which demonstrates some of the difficulties a marriage between Emma and William would encounter, as well as being a quiet tragedy all its own. Mori is also excellent at using the reactions of other people to Emma to indicate the subtle changes in Emma, and the kinds of things the very reserved Emma may feel but won't say.