Word & Void
The Word & Void series (also called 'The Word and The Void') by Terry Brooks is a trilogy of dark urban fantasy novels primarily set in Illinois in the late 20th and early 21st century. The series focuses on John Ross and Nest Freemark, forced acquaintances who use magic given to them from the Word (the embodiment of good in the world) to prevent mankind from being overcome by the demonic forces of the Void. Terry Brooks' later trilogy, The Genesis of Shannara, bridges this series with his previous Shannara novels.
Running with the Demon
A Knight of the Word
The second novel concerns John's fall from his faith and Nest's quest to restore it.
Angel Fire East
In the final novel of the trilogy, John and Nest unravel the secret of a magical creation called a "gypsy morph", which has the potential to turn the tide in the battle against the Void.