Sculduggery Pleasant. Dark Days: Derek Landy

Meet Skulduggery Pleasant: detective, sorcerer, warrior.
Oh yes. And dead.
Skulduggery Pleasant is gone, sucked into a parallel dimension overrun by the Faceless Ones. If his bones haven`t already been turned to dust, chances are he`s insane, driven out of his mind by the horror of the ancient gods. There is no official, Sanctuary-approved rescue mission. There is no official plan to save him.
But Valkyrie`s never had much time for plans.
The problem is, even if she can get Skulduggery back, there might not be much left for him to return to. There`s a gang of villains bent on destroying the Sanctuary, there are some very powerful people who want Valkyrie dead, and as if all that wasn`t enough it looks very likely that a sorcerer named Darquesse is going to kill the world and everyone on it.
Skulduggery is gone. All our hopes rest with Valkyrie. The world`s weight is on her shoulders, and its fate is in her hands.
These are dark days indeed.