The Mediums' Book is about mediumship – a faculty that enables the communication between human beings and the spirits of the so-called dead. Allan Kardec was the first to seriously study mediumship by using a scientific method. The book presents the teachings of spirits on all types of spirit manifestations, the ways of communicating with the invisible world, the different types of mediums and the development of mediumship. Employing scientific strictness, it analyzes hoaxes and charlatanism, and proposes methods of verifying and controlling the information given by spirits. Finally, it defends the use of reason in dealing with subjects related to spirituality.