Start with high-confidence angels & demons books, then narrow by subgenre or adjacent topics.
Sense and Nonsense About Angels and Demons
Kenneth D. Boa, Robert M. Bowman Jr.
The Dark Side of the Supernatural, Revised and Expanded Edition
Bill Myers, David Wimbish
Angels: a 90-day Devotional About God's Messengers
Christa J. Kinde, Christa Kinde
Lucifer
Jeffrey Burton Russell
Angels & Saints
Eliot Weinberger, Mary Wellesley
The Mystery of the Supernatural
Henri de Lubac
Angels to Help You
Lester Sumrall
A Divine Revelation of Angels
Mary K. Baxter, T. L. Lowery
Demons the Answer Book
Lifted by Angels
Joel J. Miller, Dean Gallagher
Entertaining Angels
Anne Neilson, Roma Downey
Angels
Billy Graham
Reviving Old Scratch
Richard Beck
This Season of Angels
Perry Stone, Marc Thompson
The Forbidden Knowledge of the Book of Enoch
Harold Roth
Dr. David Jeremiah, Tom Force
Paradise Regained
John Milton
A Divine Revelation of Heaven & Hell
Angels in Your Presence
Omar Suleiman
The Physics of Angels
Rupert Sheldrake, Matthew Fox