Grim Tuesday  - Garth Nix
This is the next book in his Keys to the Kingdom series, following on from Mister Monday.

In Grim Tuesday, our protagonist Arthur Penhaligon has only just returned to normality after the events of the previous book when he's summoned again - although only hours have passed in Arthur's world, much more time has passed in the House and there's a need for Arthur to intervene. In order to save his family, and the future of the world itself, Arthur must try and get hold of the next Key, the one held by the eponymous Grim Tuesday.

Supporting characters from Mister Monday also make an appearance, as Nix tries hard to keep up the pace set in the first book. This time around, Arthur discovers that there's a much darker side to the place he has been drawn to, in addition to what he already knew. The series continues in Drowned Wednesday, which I look forward to reading.