Supernatural is a shortened, less in-depth version of Heiser's The Unseen Realm. I read Supernatural before The Unseen Realm and if you aren't familiar with terms and concepts like The Divine Council, The Deuteronomy 32 worldview, and other topics like the one's Heiser talked about in both books then Supernatural is the best one to start out with, if you're already familiar with the concepts then you can dive straight on into The Unseen Realm.

I will say Supernatural is great for a quick read that is also very educational. Heiser's explanation of the Biblical cosmic worldview will broaden the mind and improve Biblical reading. I know I'm delighted when I read about books that focus on ancient biblical worldview; it helps improve my Bible reading. Supernatural is clear and concise, and not bogged down with heavy word matter. The layout of the book is very well thought-out and build upon each other, showing the point Heiser is trying to accomplish.