The last series of books is a tetralogy still under development and comprises:
- The Runes of the Earth – Putnam (US), Orion (UK), October 2004.
- Fatal Revenant – Putnam (US) October 9, 2007; Orion (UK) October 18, 2007.
- Against All Things Ending (October 19, 2010)
- The Last Dark (October 2013)