There are 9 books in the series, although only 7 are currently released. They are made up of three trilogies:
The Faceless Ones Trilogy:
Book 1: Skulduggery Pleasant
Book 2: Playing with Fire
Book 3: The Faceless Ones
The Necromancer Trilogy:
Book 4: Dark Days
Book 5: Mortal Coil
Book 6: Death Bringer
The Darquess Trilogy:
Book 7: Kingdom of the Wicked
Book 8
Book 9
The Faceless Ones Trilogy:
Book 1: Skulduggery Pleasant
Book 2: Playing with Fire
Book 3: The Faceless Ones
The Necromancer Trilogy:
Book 4: Dark Days
Book 5: Mortal Coil
Book 6: Death Bringer
The Darquess Trilogy:
Book 7: Kingdom of the Wicked
Book 8
Book 9