A Skinful of Shadows is the fourth Frances Hardinge novel I have read (one of the others, A Face Like Glass, was also a Girls Underground book), and they have all been fantastically written and noticeably unique one from the other (unlike many prolific authors who seem to just repeat the same scenarios and environments over and over).
Makepeace is a young woman living during the English Civil War, who is embroiled in a much stranger war of her own, for control of her very body and soul. She comes from a long line of people with a special gift (or curse) – a hollow space inside that can be occupied by spirits. As a child, her mother taught her to ward off the ghosts who tried to get inside her. But an impetuous decision leads accidentally to her mother’s death, and Makepeace is sent to live with the aristocratic family of her dead father, where she discovers terrible secrets behind their wealth and power. As she works to unravel the mysteries and protect herself, she begins to acquire companions – the first is her half-brother James, but the rest are all spirits who come to live inside her, including the ghost of an angry, abused bear who becomes her closest ally once she learns how to coexist with him. (The relationship she has with this animal spirit is profound and complicated and one of the best things about the story, especially as it is communicated entirely without any conversation possible between herself and the bear.)
When James is possessed by ghosts, Makepeace goes on a long and dangerous quest to find a cure, which might also save her someday. There is not one Adversary but a collective of them, manipulative ghosts who have lived forever in borrowed bodies and have set their sights on her next. Despite several painful betrayals, she manages to defeat them all with her cleverness and tenacity – although she never faces them alone per se, since with all the ghosts inside her, she will never be truly alone.