J. B. Turner argues JS is the sole author of the BOM.
J. B. Turner, Mormonism in All Ages; or, the Rise, Progress, and Causes of Mormonism (New York: Platt & Peters, 1842), 150
1. Our first point respects the character and credibility of Joseph Smith, jun., who announces himself, on the title-page of the first edition of the Book of Mormon, as "the AUTHOR and PROPRIETOR" of that work.
Our first remark is, that we cheerfully admit this claim.
We cannot conceive how any man of common sense could ever have imagined that God, or any other being, except Joe Smith, was either the author or proprietor of such a book. The only difficulty is, to see how God can be responsible for a work of which Joseph Smith is "Author and Proprietor;" and one ground on which such a claim must be sustained, is the admitted excellence and trustworthiness of Joseph Smith's moral character.