Book of Mormon
According to Mormons, it is "another testament of Jesus Christ," written by the ancestors of the native Americans. The Book of Mormon claims that native Americans are descended from Jews, who escaped the destruction of Jerusalem and crossed the sea to populate North and South America. This tome is a saga that spans from approximately 600 BC to AD 421.
The Book of Mormon has 15 books, named for their alleged authors, which are divided into chapter and verses. It is written from the perspective of prophets who handed the book down through the generations. The style is similar to Early Modern English, used for the King James Version of the Bible, and borrows at length from biblical authors such as Isaiah. In fact, the book of 2 Nephi contains large passages quoted verbatim from Isaiah (2 Nephi 11-24, Isaiah 7-14 ). The first English edition required thousands of corrections, including bizarre inconsistencies. 
A likely explanation is that Joseph Smith was the sole author and not the translator of the Book of Mormon.
It is also quite possible that the Book of Mormon was written by multiple authors in the 19th century, possibly including Sidney Rigdon, Oliver Cowdery and Joseph Smith.
According to the church, the Book of Mormon was written approximately 600 BC to AD 421 by the prophets named in the text. They were then compiled and abridged by the prophet Mormon into gold plates from the earlier prophet's records which were engraved on metal plates. His son Moroni added a final chapter and hid the gold plates in a hill Cumorah near what was later called Manchester, New York.  These original plates were supposedly found centuries later by Joseph Smith who was directed to them by Moroni, who appeared as an angel, on September 22, 1823.
Smith claimed the language of the original text was "reformed Egyptian." Joseph described the plates thus: "...each plate was six inches wide and eight inches long and not quite so thick as common tin... The volume was something near six inches in thickness....”  He claims to have taken the plates back to his home in 1827. Smith told his followers and family that he was forbidden to show the plates to anyone, or they would be stricken dead. Joseph probably got the idea for buried metal plates from a story that appeared in a Palmyra paper in 1821, describing the discovery of brass plates found in Canada.  Several witnesses, all members of the church, claimed to have seen the plates.
Smith claimed Moroni instructed him in the use of the Urim and Thummim, seer stones which Smith used to translate the tablets. His translation of the gold plates began in 1829. He allegedly used the Urim and Thummim to translate them, but eyewitnesses claimed that Joseph used his other seer stones, including the ones he used during his money-digging exploits, for translation.  Oliver Cowdry claimed that, during the translation of the plates, the plates wouldn't even be present. Joseph would just look into the Urim and Thummim and dictate, taking frequent breaks and walks down by the river to skip stones (Ch 4, pp 61).
Conveniently, the plates were not shown to non-church members and supposedly returned to the angel Moroni.
The book tells of a group of people escaping Jerusalem before its fall to the Babylonians in 586 BCE. They journey to the Americas by boat. The group split into two tribes, the Nephites and the Lamanites, who often warred with each other. Jesus is said to have returned from heaven after his ascension and visited the tribes in the Americas. He established a peaceful and prosperous society, however this reverted to its original state within a few generations. After records of these events were hidden, the Nephites were destroyed, leaving only the Lamanites (who are now known as the Native Americans).
Book also tells of a group called the Jaredites, who travelled from the Tower of Babel to the Americas around 2500 BCE.
It is likely that Joseph Smith got his ideas for the story of the Book of Mormon from several contemporary sources, including Ethan Smith's book View of the Hebrews.  Ethan Smith's book speculated that Native Americans were descended from Hebrew settlers. The view was popular among scholars at the time. 
The View of the Hebrews also compared copper breast plates found in New England burial mounds with the ephod (part of the ceremonial garb) of Hebrew high priests, including the Urim and Thumim. This may also be what spawned the breastplate and seer stones that constituted the Urim and Thumim which Joseph Smith sometimes used when "translating" the gold plates.
The Holy Spirit tells Nephi to kill Laban in order to recover a tablet with genealogy information, which Nephi proceeds to do. (1 Nephi 4:18) Is this the best an all powerful God can do? Perhaps providing a new copy would have been less murderous?
The Book of Mormon contains many blatant errors. It describes a large society with many large cities in the Americans; no evidence for this has been found. Also, it mentions many uses of advanced technology, such as the use of steel. Animals such as horses are said to have existed in the Americas but there is no other evidence they existed before the arrival of Columbus in the 15th century CE. The lack of human inhabitants at the time of the arrival of the Jaredites is also incorrect; evidence exists that humans have continually lived in the Americas for 16,500–13,000 years.
No such language as "reformed Egyptian"
There is no evidence that the language reformed Egyptian exists. If it existed, we would expect to see other extant examples which are independent of the church.
Divination was used in translation
Joseph Smith claimed he used divination to translate the Book of Mormon using Urim and Thummim and his other seer stones. This method has no evidence for its validity. Joseph Smith could have attempted to translate a widely available text in a language that was unknown to him to prove he could perform the feat; this was not done probably because he was a fraud.
According to a former president of the church, Joseph Fielding Smith, they still possess one of Joseph Smith's original seer stones.  If they want to provide evidence of their validity, they would demonstrate they still work (under scientifically controlled conditions). The James Randi Educational Foundation would be happy to help them organize a suitable demonstration (and pay a large prize of they are successful). However, the church will not do this because divination is a fraud.
No original copies survive
If the Book of Mormon was written between 600 BC to AD 421, we would expect to copies or originals of the source material Mormon used in the compilation. No such copies exist. There is no reasonable explanation why the writings were to be kept secret or they were completely lost, the collapse of the Nephite civilization notwithstanding. Therefore, the Book of Mormon was not based on pre-existing writings (and more than likely it was not written before 1829). There is also no original copy in the original "reformed Egyptian" language. If Joseph Smith really had a new testament, it would have been wise to copy the text without translation and translated it once this was complete.
Weight of the gold plates
Scholars have pointed out that, if Smith's description were true, the gold plates would have weighed around 200 pounds. This contradicts several other stories, including one which claims Joseph Smith ran with the plates while fending off assailants.
Joseph Smith was an admitted con man
- Main Article: Joseph Smith was a con man
The authenticity of the Book of Mormon depends on the credibility of Joseph Smith. However, he admitted to perpetrating various gold digging, divining and necromancy activities.  Most accounts describe Joseph Smith actually testifying under oath that he really could divine the location of treasure! If he thought he could divine treasure with a seer stone, he was self-deluded; if it was knowingly deceiving people, he was a con man.
Missing 116 pages
After 116 pages of the Book of Mormon had been transcribed, Martin Harris asked Joseph if he might take copies of those pages to his wife, Lucy. Lucy was skeptical of Smith's claims about the gold plates and wanted to see the work for herself. Smith agreed to let Harris take the 116 pages home to Lucy. Upon receiving the pages, the pages supposedly disappeared. Mrs. Harris told her husband that, if Smith was genuinely translating, then he should be able to reproduce the pages.
Harris returned to Smith with great trepidation and told him about the theft. Smith was furious. He spent two days in secluded prayer. When he emerged from prayer, he told Harris and Emma Smith, that an angel had appeared before him and told him not to re-write the missing 116 pages. The angel had instructed him to cease work on the "Book of Lehi" and instead write the "Book of Nephi". The Book of Nephi would be similar to the Book of Lehi, but since they were written from different points of view, there would be a few differences. Smith thus side-stepped being exposed as a fraud.
The first printing of the Book of Mormon in 1830 contained a preface written by Joseph Smith. This preface stated an explanation of the theft of the 116 pages, and framed their loss as attempts by the devil to thwart God's plans to get the Book of Mormon published.  
Not consistent with the New Testament narrative
The narrative presented in the Book of Mormon is not consistent with the New Testament. The return of Jesus was supposed to bring the apocalypse. The New Testament and the authors of the early Christian church make no mention of other Christians living in the Americas.
Humans becoming angels
Some Christians and most skeptics would take issue with the idea of a human becoming an angel. 
- ↑ 
- ↑ 
- ↑ Joseph Smith, Times and Seasons, v3:9, March 1, 1842, 707.
- ↑ 4.0 4.1 4.2 Brodie, Fawn. No Man Knows My History. New York, 1945. Vintage Books: New York, 1995. ISBN: 679-73054-0
- ↑ 
- ↑ Ethan Smith, View of the Hebrews. The concept of Native Americans as descendants of the Hebrew people was common at the time, but was soon replaced with the theory of migration from east Asia (Ch 3, pp 46).
- ↑ Joseph Fielding Smith, Doctrines of Salvation (Salt Lake City: Bookcraft, 1954–56) 3: 225.
- ↑ Christopher Hitchens, Mormonism: A Racket Becomes a Religion, 2011
- ↑ Joseph Smith added a preface about the missing 116 pages (Ch 4, pp 56)
- ↑ 
- Text of the Book of Mormon on the church's website.