Those Invisible Spirits Called Angels by Renald Showers
This is an excellent book dealing with the general subject of angels from a biblical perspective. Showers discusses both holy and fallen angels, but does not mention the spiritual warfare movement. If you are interested in that subject you might want to turn to Overrun by Demons authored by Thomas Ice.
While I found this volume sound and helpful Showers does give two questionable accounts of supposed angel experiences in modern times (e.g. pp.12-14). These types of incidents cannot be verified or documented. Whether they were angel encounters, or not, is complete conjecture and has no place in a work that is otherwise based upon Scripture. Other than these two illustrations I would heartily recommend this book.