One of my students at Lincoln Christian University was asking about books on the topic–Spiritual Warfare. This is a topic I like to study. If you are unfamiliar with the topic, you should begin by reading Ephesians 6.
Here are books I enjoy in this area. Some might surprise you:
The Lord of the Rings by J. R. R. Tolkien
The Screwtape Letters by C. S. Lewis
Powers of Evil: A Biblical Study of Satan and Demons by Sydney Page
Three Questions about Spiritual Warfare by Clifton Arnold
The Chronicles of Narnia by C. S. Lewis
Pilgrims Progress by Bunyan (get a modern edition)
God at War by Gregory Boyd
The Unseen Realm: Recovering the Supernatural Worldview of the Bible by Michael S. Heiser
Seeing the Unseen: Preparing for Spiritual War by Joe Beam
Our Struggle by G. Steve Kinnard
The King Jesus Herald 