Allan Kardec's supreme handbook about the world of the spirits and the movement of Spiritism is a valuable and insightful guide to existences beyond our corporeal realm. Kardec begins this work by delving into the historical significance of spirits, their connection with God and the physical universe. How spirits are situated in the world, and how they move unseen between the planes of existence, is discussed. The second part concentrates on The Spirit World in greater detail. The types of spirits and how they are incarnated are detailed, with Kardec proposing that several distinct planes of existence are extant, and that most individuals proceed through life without knowing or seeing their influence. A chapter is devoted to how spirits might intervene in our everyday, corporeal world. Often these excursions have a purpose; the spirits are on a mission to inform someone, or change something, that harmony may be restored.

Rudolf Steiner's superb thesis provides deep insight into spiritual science, and the history of mankind as viewed through the philosophy of the anthroposophy movement he founded. An Outline of Esoteric Science attempts to reconcile mankind's spiritual being with the scientific exactitude which had emerged among scholars in the 19th century. Steiner lays out the spiritual realms which are invisible to us, attempting to use a defined precision similar to that which had emerged in science. In the final section, this book refers to the spiritual development and contemplation necessary for individuals to see the spiritual realms and planes which comprise existence and the universe. The means by which individuals may train themselves introspectively to see are detailed by Steiner, whose theosophical philosophy was, by the time of this book's publication in 1909, well-developed.