High Quality Content by WIKIPEDIA articles! The Spalding–Rigdon theory of Book of Mormon authorship is the theory that the Book of Mormon was plagiarized in part from an unpublished manuscript written by Solomon Spalding. This theory first appeared in print in the book Mormonism Unvailed, published in 1834 by E.D. Howe. The theory claimed that the Spalding manuscript was at some point acquired by Sidney Rigdon, who used it in collusion with Joseph Smith, Jr. to produce the Book of Mormon. Although publicly stated that it was through reading the Book of Mormon that Rigdon joined the Latter Day Saint movement, the Spalding–Rigdon theory argues that the story was a later invention to cover the book`s allegedly true origins.