What does "a deeper mystical side" mean? does "on the side of" mean "supportive"?

What does "a deeper mystical side" mean? does "on the side of" mean "supportive"?

A deeper mystical side of the visions of the Great War centres round the "Angels of Mons." Mr. Arthur Machen, the well-known London journalist, wrote a story telling how English bowmen from the field of Agincourt intervened during the terrible retreat from Mons. But he stated afterwards that he had invented the incident. But here, as so often before, truth proved fiction to be a fact, or at least facts of a like character were reported by a number of credible witnesses. Mr. Harold Begbie published a little book," On the Side of the Angels," giving much evidence, and Mr. Ralph Shirley, editor of The Occult Review (London), followed with "The Angel Warriors at Mons," in which he added to Mr. Begbie's testimony.

"The History of Spiritualism," by Sir Arthur Conan Doyle