It's almost impossible to give this book a rating. As a stand alone in the series, it doesn't work because it is the last. As the last book in the series, it's also a history of reading this author since before I went to high school and was just forming my hightop wearing, Moleskine carrying, nautically obsessed high school persona all the way through to awkward Thanksgivings at relatives houses, and finally going back to when I was first reading these and finishing the last one still on the train to Portland, like I started the first ones. It's really a book series beyond rating because it's one of those formative, chance encounters that fired up my nascent Anglophile nature and my preteen need for more badass lady-types (oh, if Lumberjanes had existed then!). This last volume in the series also scared the crap out of me more than any other book I've read recently (I guess a testament to the enduring nature of YA even on "adult" readers) and while I would have finished this differently (because I have Opinions when so much has happened while the series was going on, and on, and on, and on) I also know this is the end. The author, L.A. Meyer, died before publication, so this is really the end. As such, it was handled well, in the spirit of the series but also with the knowledge there will be no Harry Potter like resurrection several years down the line.
Really, the testament to this series is that when I went to London and saw my first Goya painting in person, I wondered if it was one of the ones Jacky supposedly modeled for. That when I saw Blackfriars Bridge on a map, I thought of her. Whatever issues with the technical aspects of the writing or the long term plotting, the story grabbed me from the beginning and has only now started to let me go.