Rivail wrote under the name "Allan Kardec", allegedly following the suggestion of a spirit identified as "Truth". On 18 April 1857, Rivail (as "Allan Kardec") published his first book on Spiritism, "The Spirits' Book", comprising a series of answered questions (502 in the first edition and 1,019 in later editions) exploring matters concerning the nature of spirits, the spirit world, and the relationship between the spirit world and the material world. This was followed by a series of other books, including "The Medium's Book", "The Gospel According to Spiritism", "Heaven and Hell" and "The Genesis According to Spiritism", and by a periodical, the "Revue Spirite", which Kardec published until his death. Collectively, the books became known as the Spiritist Codification.