Garth Nix's Lady Friday is probably the darkest of the "Keys to the Kingdom" series so far. Arthur Penhaligon's adventures in the House of the Architect are becoming more dangerous as he nears the end of his quest to claim all seven of the Keys to the Kingdom. Four of the seven Trustees have now been defeated and their Keys claimed by Arthur, with the relevant parts of the Will amalgamated into Dame Primus, but for Arthur things are just getting worse. He's turning into a Denizen, instead of remaining an ordinary Mortal as he wishes. His friends Suzy Turquoise Blue and Fred Initial Numbers Gold were captured by the Piper at the end of the last book, and they're missing, presumed under the control of the Piper, whose New Nithling army still controls most of the Great Maze.
To make matters worse, Superior Saturday has turned off all the Elevators in the House and has ordered that the Front Door be locked, so Arthur can't get home or find out what is happening there. All he knows is that Leaf isn't on Earth any more - she's missing, along with hundreds of other people who were transferred from regular hospitals in the Secondary Realms to a private institution run by a "Doctor Friday" (aka Lady Friday). From there they've been taken to somewhere else in the Secondary Realms. In addition to all this trouble, Arthur's mother is also missing, and he must weigh up an offer from Lady Friday that is either a cunning trap for the Rightful Heir or a golden opportunity he must seize - before either Superior Saturday or the Piper (who've both been made the same offer) can beat him to it.
Arthur finds himself working with a Newnith (as the New Nithlings have styled themselves) in a rather uneasy alliance, and racing to find the Fifth Part of the Will before Superior Saturday or the Piper can claim the Fifth Key. In the meantime, Leaf is trying to work out how to contact Arthur, and how to stay alive when Lady Friday is determined to use her as a hostage. To make things worse for Leaf, her Aunt Mango is also being held hostage by Lady Friday. And Superior Saturday has declared all the Piper's Children are to be killed on sight, which means Leaf is in even more danger than Arthur, who at least has the Fourth Key still to aid him.
The series is building up to an epic, climatic battle in the sixth book. It will be interesting to see how Arthur gets on in the final book, since Lord Sunday appears to have handed over most of his power to Superior Saturday.
Lady Friday is also available from Amazon.com.