What is the sixth skulduggery pleasant book called?

Skulduggery Pleasant: Mortal Coil (September 2nd, 2010)

The comparatively short time period since Dark Days (April 1, 2010) is due to the fact that a cliff-hanger of sorts exists in the 4th book, to be resolved in the 5th book.
Skulduggery Pleasant 5 is called Mortal Coil.