This Book titled "Biblical Demonology" by Dr. Merrill F Unger, Professor at Dallas Theological Seminary is probably the best and most comprehensive book on the subject; excepting the one on "Christian Counselling and Occultism" and "Occult Bondage and Deliverance" - both by Dr. Kurt E Koch.
Both the above mentioned authors are experts in the field of Biblical Demonology; they both understand well the Scriptures and are sound in their teachings. Only the Bible has the true answers to man's problems - demon possession or to be demonized is definitely a problem that no one can solve without the Almighty Power of our Lord and Saviour Jesus Christ. True freedom from demonic and Satanic oppression is only available through the power of God, our Lord Jesus Christ.
I whole heartedly recommend both the above books for anyone and everyone who wants to understand the subject. There are of course many other books published by good Christian publishers, but the above mentioned authors I believe are the best and most authoritative to read. And I use the word authoritative only because they totally rely on the 'authority' of the Word of God - the Bible; which alone is the 'touchstone' by which all Truth and/or facts are to be and should be judged. And therefore because the above mentioned authors rely so very completely on the Word of God, I believe they have arrived at the truth of the subject.
One other book by Dr. Unger "Demons in the World Today" is also very good; besides the one he wrote entitled "What Demons can do to saints".
I could also recommend the following good sound Christian books -
1. "The Invisible War" by Dr. Donald Grey Barnhouse
2. "Satan is no Myth" by J Oswald Sanders
3. "Angels, Elect and Evil" by C Fred Dickason
4. "Demon Possession and the Christian" also by C Fred Dickason
Of the above listed, the last one by CFD is more comprehensive and most interesting. Not all will perhaps agree with the above listed authors and their conclusions. But they each are solidly 'evangelical' in their approach to the Word of God and therefore definitely worth reading.
Finally, above all, read - meditate - live by - the Word of God (The Bible). This alone will keep us in the Love of our Lord Jesus Christ and by His Grace alone can we each live for Him.
And now a word to those who are not believers in our Lord Jesus Christ. Demonology is definitely not a subject to be 'dabbled' in; no, not out of curiosity. If you dear reader are having troubles with Satanic 'forces' you need to come to the Lord Jesus Christ who alone can deliver you. However, if you are not a 'Christian' - and by Christian I mean you not are one who has committed your life to the Lordship of Jesus Christ, you have not owned Him alone as your Lord and Saviour - you are not yet 'born' from above; you are not born of God into His family - you need firstly and most importantly to believe on Him. You have to first settle the 'sin' question in your life, before you can experience deliverance from the Devil.
The Bible says that all those who believe in the Lord Jesus Christ are born of God and all such God transfers them from the kingdom of Darkness into the Kingdom of His Son - our Lord Jesus Christ.
And such who are children of God, they keep themselves in the Love of God and by God's grace and power alone, the Evil one does not 'touch' such as these.
May you all find true love, joy and peace in our Lord Jesus Christ