Arthur Penhaligon's week just keeps getting worse. The Keys of the Kingdom series hits a turning point in the fourth book, "Sir Thursday." People are killed, new creatures appear, and Arthur finds himself in a lethal power struggle with the Morrow Days. Garth Nix is in fine form in this book, one of the best of the series, and it only promises to get better. When Arthur and Leaf try to return to Earth, they are stopped by Dame Primus -- it seems that a Spirit-Eater (the Skinless Boy) has taken Arthur's place on Earth. If he goes back home, he might destroy the world. To make matters worse, he is tricked into accepting Sir Thursday's shilling, which means he's been drafted into the army... for a hundred years. Leaf goes back to Earth, and tries to destroy the Skinless Boy, with the unexpected help of Suzy. Meanwhile, Arthur struggles in the Army -- especially since part of it is being "washed between the ears." As he tries to remember to remember who he is, Arthur becomes a part of the regiment fighting a new breed of Nithling -- and under the command of the berserk Sir Thursday. Unexpected twists are the order of the day on "Thursday" -- Garth Nix serves up mind-reading spores, assassinations, new Nithlings, and the Piper (often mentioned, but never before seen). With a stunning ending and some otherworldly battles, this is probably the best book since "Mister Monday." And Nix's detailed, dark-edged writing is given a full workout here.He does a great job of describing battles against Nithlings and Nothing, as well as the creepy invasion into our own world. It takes awhile for Sir Thursday to even become part of the plot -- much of the book, actually -- but when he does show up, he's a suitably nasty Day. Think a demented drill sargeant. Since Arthur has already come to terms with being the Heir, Nix lets him focus on something even scarier -- using the Keys is slowly turning him into a Denizen. Since he has amnesia for a good chunk of the book, he's most afraid near the end. And Leaf gets some further fleshing-out, as she tries to destroy the Skinless Boy without being taken over by it. Perhaps the worst part of it is that "Sir Thursday" ends on a double cliffhanger. It's going to be awhile before we're introduced to Lady Friday, but "Sir Thursday" was worth the wait. Chilling and wonderfully dark.