Demon assault can be very real. They took on Paul and Jesus, so it's not necessarily a matter of some sin that's giving entrance, as many say.

I have read books by Christians who offer insights on demons that go way beyond Scripture. Usually this is based on personal experiences. Some have gotten a good deal of “information” from being present at exorcisms.

From The Ishbane Conspiracy by Randy Alcorn, Angela (Alcorn) Stump and Karina (Alcorn) Franklin  Dear Foulgrin,I’m pleased our favorite holiday’s coming up in a few weeks. Halloween! It’s official, Foulgrin—these vermin now spend more money on this holiday than any other except Christmas.

To those wishing to explore a biblical perspective on demonic nature and activity, as well as spiritual warfare, Randy recommends these books: 3 Crucial Questions about Spiritual Warfare, by Clinton Arnold (Baker Books, 1997). Lord Foulgrin’s Letters, by Randy Alcorn

Most people show by their priorities and their casual approach to spiritual things that they believe we are in peacetime, not wartime.

From Lord Foulgrin’s Letters, a novel by Randy Alcorn
(inspired by C. S. Lewis’s The Screwtape Letters)

Diane’s conversion took my breath away. Only Elyon’s Spirit can pierce the hard shell of their hearts. When she accepted His gift and asked His forgiveness, Elshar and I embraced.

It is unthinkable to me that God would allow one of His children, whom He purchased with the blood of Christ (1Peter 1:18-19) and made into a new creation (2Cor 5:17) — to be possessed and controlled by a demon.  

This is a question and answer concerning prayer and angelic warfare portrayed in Randy Alcorn's books Deadline, Dominion and Edge of Eternity.