Lord Sunday by Garth Nix

As Arthur Penhaligon seeks to find the seventh key, complete his quest to save the Kingdom to which he is heir, and learn his true identity, he discovers that he has a greater purpose than he could have imagined.

I'm still not quite sure how I feel about this book & the ending. It was very strange and very unexpected. Overall, a lot of the characterizations that were being built up were never really explained. There weren't any actual loose end; however, the conclusion felt pretty unsatisfying.

In any event, The Keys to the Kingdom books stand alone very poorly; by the seventh book every character, item and place has significance from earlier on in the book and there is no summarization of previous events. I would recommend re-reading the entire series before starting this.