Psychic Self-Defense by Dion Fortune. http://www.sacred-texts.com/ has a lot of free online books, but I can't seem to find that one specifically.
It also depends on your interest. This is my favorite online course in witchcraft: https://thebookoft.wordpress.com/knowledge/witchcraft-from-the-ground-up/preface/
But it's not Hermeticism, or Seidhr or Galdhr, or British Traditional Witchcraft, or Theosophy. Those are different branches.