His extensive involvement in researching the supernatural included investigating The Amityville Horror and some of the most prominent haunted locations around the world. He also worked with well-known trance mediums such as Ethel Johnson-Meyers, Sybil Leek, and Marisa Anderson. Holzer has been credited with creating the term "The Other Side" (already in use, however, in nineteenth-century spiritualism) or in full "The Other Side of Life". He is also sometimes credited with having coined the term ghost hunter, which was the title of his first book on the paranormal published in 1963. However, an earlier book by Harry Price published in 1936 was titled "Confessions of a Ghost Hunter".