Derek Landy 
Death Bringer 

Meet Skulduggery Pleasant: detective, sorcerer, warrior.Oh yes. And dead.The Necromancers no longer need Valkyrie to be their Death Bringer, and that’s a Good Thing.There’s just one catch. There’s a reason the Necromancers don’t need her any more. And that’s because they’ve found their Death Bringer already, the person who will dissolve the doors between life and death.And that’s a very, very Bad Thing…