John Gilhooly’s book on this topic has a self-explanatory title. One of my favorite things about this book is that it is entirely Word-centered. As he says on the first page, “Perhaps the most troubling tendency in literature on spiritual warfare, however, is the extent to which mere anecdote bears the brunt of the heavy lifting.” Instead of telling stories of his own spiritual conquests and confrontations with demons, Gilhooly’s book is truly helpful by being focused and centered on the Biblical text, not anecdotes. If you are looking for a level-headed book that will answer your basic question about the angelic and demonic, this may be the best place to start.