Who wrote the Book of Mormon?
No one can say with certainty who wrote it, or which author wrote which portions. It would literally take me days to write out the evidence which leads me to my current conclusions, time that I don't have. The evidence is out there, if you care to study it. However, if your prerequisite goal of such a study is to remain a Mormon, then I daresay any evidence that would lead you away from that predetermined goal would be useless to you. The first premise that I reject is that the BOM is an ancient work, because of its insurmountable mountains of anachronisms which identify it as a modern effort, placing it squarely in the culture of the 1820s, and plagiarism from other widely available works.
Therefore, I leave any idea of its "authenticity" behind from the outset. I next take a long, hard look at the church which grew from Joseph Smith. Having been an active Mormon for 35 years, and semi-active for another seven, I feel I have a pretty good picture of the LDS church, its history, and motives. The next step is to study the background and motivations of those responsible for its production: Joseph Smith, Emma Smith, Martin Harris, Sidney Rigdon, Oliver Cowdery, etc. Having read tens of thousands of pages of material, pro and con, here in a nutshell is my current "best guess", included with a preface that I don't care what anyone has to "refute" my position; I've read both sides many times over, and I can argue the pro-Mormon side just as easily as the no-Mormon side, with the prime diference being that the pro-Mormon side doesn't make any sense.
Sidney Rigdon was excommunicated as a Baptist minister on October 11, 1823, for teaching false doctrine, in Pittsburgh, Pennsylvania. He was already sort of an eccentric character, having a brain injury from falling off a horse as a youth. He gravitated to Alexander Campbell's Disciples of Christ, and was a Campbellite preacher until he "converted" to Mormonism in 1830. We've all heard the story of Solomon Spaulding, that he wrote several "romances", and that at least one of them was written in "the ancient style", according to witnesses.
There is much corroborative evidence that Rigdon somehow procured one of Spalding's manuscripts from a print shop in Pittsburgh, at the time he lived there in 1822 or 23. (There are as many witnesses to the similarity of Spalding's manuscript to the BOM as there are witnesses to the "gold plates.") There are also witnesses who claimed that Rigdon was an acquaintance of the young Joseph Smith, and had been seen in his company in Palmyra before the BOM was "translated"--- that Rigdon wasn't "converted" in late 1830, but was well aware of what was going on---that he had given the charismatic JS the material with which to produce the BOM, and that Rigdon relegated himself to a number two role, disguising his involvement. That would easily explain why JS moved his congregation to Kirtland to join Rigdon's immediately after the church's founding, and would also explain the many dubious financial endeavors Smith and Rigdon undertook together, including the United Order and the Kirtland Bank fiasco (which got them run out of town)---their motive was to build a financial empire based on their religous commune, with a side motive of Rigdon's being his dismissal from the Baptist faith, and holding a grudge.
Joseph Smith was a born talker and charismatic, with a vivid imagination. He was introduced to the world of showmanship by a traveling conjurer known as "Walters, the magician", who passed through Palmyra in the early 1820s. JS took Walters' conjuring (or "divining") as his own, using a divining rod and his peep-stone to search for buried items as early as 1826 (the year he was arrested and tried, on March 20, at age 20). Some witnesses claimed that the Smiths had been "money-digging" since at least 1820, and JS found and began using his "seer stone" in 1822. Compounding his money problems was the fact that his family owed a balloon payment on their newly-built home, and were about to lose it. This prompted them to engage in a get-rich-quick scheme of digging and selling ginseng root, sending it to China by a courier. The courier sold it, kept their money, and the Smiths eventually lost their home as a result. Thus, in 1826-29, JS was motivated by the need of a large amount of money in a hurry.
Oliver Cowdery was Joseph Smith's distant cousin, whose family's minister was Ethan Smith of Poultney, Vermont, author of the 1823 "View of the Hebrews." The official story, of course, is that Smith and Cowdery had never met, and that Cowdery just happened to get a job teaching school in Palmyra and boarded with the Smith family. My theory requires that Smith actually hired Cowdery to put Rigdon's manuscript to paper, inserting Rigdon's Campbellite Restorationist theology, which Rigdon would have had some five years (1823 to 1828) to adapt. Cowdery brought with him a copy of "View of the Hebrews" with which to add filler material on both historical and religious portions. Cowdery's motive was money, as illustrated by his going to Toronto immediately after the BOM's publication to try to sell its copyright for $5,000; the fact that it was Cowdery, not Smith, who took the BOM manuscript pages to the printer; and also power, as JS made Cowdery the "second elder" of the church upon its founding.
Before Cowdery arrived, JS attempted to use both Emma and Martin Harris as scribes, but JS could not write well, and soon discovered that neither of them could either. Thus the fact that the "translation" went very slowly until Cowdery arrived, after which the entire work was finished in about six months. I haven't made up my mind whether Harris was a dupe or not. There is evidence both ways, including a statmement from his sister Abigail, that MH told her "What if it is a lie; if you will let me alone I will make money out of it!" The same is true of the Whitmers. I know that the Mormon line is "none of the witnesses ever retracted their testimonies", but there are also recollections to the contrary. The fact that so many Whitmers were "group" BOM "witnesses," and that ALL of them had left Mormonism by 1842, is suspicious. As to the "wordprint analysis" issue, that would be explained by so many authors having "irons in the fire." Rigdon's Campbellite exhortations; Spalding's romantic style, repeated "and it came to pass", and "wonder tales"; Ethan Smith's ideas that Indians were displaced Hebrews, and his wide use of Isaiah, which BH Roberts covered well; ample evidence that the Apocrypha was used for the BOM's production, including the name "Nephi" (the Tanners have done thorough research on that line); and Cowdery's writing ability, which put it all together.
Naturally, a group of people who are intentionally writing a work in the second-hand voice of different people throughout the history, would be inclined to change their writing style accordingly. Another big blow to the entire story of the gold plates, angel, etc., is Dr. Reed Durham's speech "Is There No Hope for the Widow's Son," wherein he discussed the similarities between JS' "Moroni" story and the Masonic legend of the faithful "Hiram Abiff"; the cave, the stone box, the treasure, etc. JS was VERY familiar with Masonic traditions, and is very easy to theorize how he co-opted that existing story to explain his discovery of the "gold plates."