Generally a reference to the Book written by St. John, which is the last book of the Christian Bible. This book is also called the Revelation of St. John. This controversial and deeply symbolic work is discussed in many of the books of Samael Aun Weor, but especially in The Aquarian Message and The Revolution of Beelzebub.
"The fundamental book of the Great Work, which James carries in his hands, is the Apocalypse (Revelation). Unquestionably, the Apocalypse is the book of Wisdom, which is comprehensible only by the Alchemists. Only the workers of the Great Work can comprehend the Apocalypse. The secret science of the Apocalypse is found in the superior Chemistry, which is Alchemy. The laws of the superior Chemistry or Alchemy, the principles, the order of the Magisterium of the Fire, are found placed in the Apocalypse." - Samael Aun Weor, The Gnostic BIble: The Pistis Sophia Unveiled