BACKGROUND: The "Spalding-Rigdon theory" was first given wide publicity in 1834 with the publication of the book Mormonism Unvailed. In it, the author followed up on indications previously published in Ohio-based newspapers that a man named Solomon Spalding was the real (and unwitting) originator of the Book of Mormon (this is in contrast to some LDS apologists who claim that the author simply invented the theory).
To make an extremely long story very, very short, the theory goes that Solomon Spalding authored a work called "Manuscript Found" (not to be confused with "Manuscript Story"!) but died while it was still at the publishing house. Supposedly Sidney Rigdon, who actually lived nearby and would later become Joseph Smith's right-hand man, was acquainted with the publisher. Rigdon procured Spalding's manuscript, edited it by inserting a great deal of religious material, and later turned it over to Joseph Smith who then used it, and not any golden plates, as full or partial source material for the Book of Mormon.
The Spalding document described in Mormonism Unvailed was rediscovered in Hawaii in 1884, but it was shown to be far from the expected word-for-word parallel to the Book of Mormon, so the theory went into hiatus. However, it must be noted that the recovered document, arbitrarily dubbed "Manuscript Story," contains plenty of strikeouts and additions. Not only that, but the document ends with a sudden cut-off in mid-sentence, suggesting that the recovered document was, at the very most, only a first draft, and that the final version, "Manuscript Found," is still missing.
The theory went into virtual moratorium in 1945 with the publication of Fawn M. Brodie's No Man Knows My History. Although considered by Mormons as an anti-Mormon book, she rejected the "Spalding-Rigdon theory" of Book of Mormon authorship due in part to her documentation of Rigdon's activities at the time which supposedly proved that Rigdon never met Joseph Smith until after the Book of Mormon was published. She had, however, imported this information wholesale from a pro-LDS source and had done little, if any, of her own research into the matter. There are also significant gaps in the chronology.
Although the Spalding-Rigdon theory was on the back burner for over half a century, in July 2005 the book Who Really Wrote the Book of Mormon? The Spalding Enigma hit the shelves. It contains a great amount of brand-new evidence and will very likely bring the Spalding-Rigdon controversy back to "front and center."