I personally believe the book's official plot description to be really misleading, because it makes you believe that Lily is an evil person, which the book proves from the very start that she is frustrated about her situation, more than anything else.
That said, this is the first time I have read a book starring: conjoined twins! Just that Lily died when she was an infant during surgery to separate her from Rose's lower back, but her soul is stuck to Rose's body and fully incapable of communicating with anyone. Not even ghosts can see her.
Lily has lived this way for Rose's entire life, and she's pretty fed up seeing the world from Rose's back. She can see and hear everything that goes on, but she can't smell, touch or taste anything, much less be able to share her thoughts (which are expectedly riddled with age appropriate swearing).
Rose of course doesn't know any of this. She has lived her entire life with her dad in the English countryside sometime in the 1950's, and would have continued to live without knowing about her sister's plight until two classmates taunt her to accompany them one evening to practice some spells from an occult book. Most of the wacky spells don't work, but Rose recites the incantation to make spirits visible, and what do you know? Lily's tethered ghost appears and Rose can hear and see her for the first time!
After recovering from the shock on behalf of both startled parties, Lily suggests that they should recite a spell that could open the portal to the spirit world in order to liberate Lily from her imprionment. Little did they know that they would enter the netherworld of Killian and enjoy plenty of wacky adventures in the search of their deceased mother who could be the only person capable of separating them.
I really, really liked the worldbuilding. Killian is just this strange planet with little countries inspired by different Earth eras where dead humans live a temporary second life. Sadly the bewildered sisters can't enjoy a tour of their strange surroundings because both humans and incomplete spirits (derisively called primitives) aren't welcome and the current ruler of this world usually executes them. Most of the book is a wild goose chase where the girls meet eccentric friends such as Pei, Stanley, and a small group of petty criminals transformed into human sized ayes, while the governs and bounty hunters are chasing them wherever they go.
Rose is bashful, whereas her sister Lily is very sassy and with a lot of attitude issues. Lily starts to develop a softer side when she discovers she can do amazing things Rose can't in the spirit world (I won't spoil it though). I really enjoyed Stanley's character. He's just really funny and stubborn. The Irish brothers are also interesting. The chapter that delves into their human life paints them as victims, but they now live in Killian as ruthless mercenaries.
All in all, the book really surprised me by how much fun it was to read it in one go, and had it not had the misfortune of being placed in a highly competitive group in this year's spfbo, it would have easily made it to the semifinals of its assigned blog.
Even if YA isn't your favored genre (given the sisters are 9 years old, I think this book is technically middle grade although a mightily darker one), this book has all of the adventure and none of the heroine does insanely stupid things, boring Mary Sue tropes or something that a lot of people hate: teenage love triangles. This is a story of sisterly love that was very fun to read!
(review of free book)