The Book of Mormon is an extra-Biblical text of Mormonism. It is claimed that Joseph Smith, the founder of Mormonism, translated golden plates and the result of this work was the Book of Mormon: Another Testament of Jesus Christ (hereafter BoM). First published in 1830, the BoM has gone through many changes involving grammar, name corrections, and even some doctrinal changes. The BoM is considered as inspired scripture in the Mormon church, and thus to LDS has equal, and arguably more authority than the Bible.
The BoM claims to be a translation of the historical record left by Hebrew descendants known as the "Nephites" who lived somewhere in the Americas between about 600 B.C. and 420 A.D.. With regard to theology, the BoM generally reflects the Protestantism of the early 19th century in America, and contains very few of the doctrinal distinctives found in the mainstream sect of Mormonism. This makes the BoM a key component of Mormon marketing. It is freely distributed and is designed to be the first exposure that many get to the largest and most popular sect of the Mormon religion.
Most Mormon apologists and many laymen, in response to DNA research, BoM demographics, and the Native American traditions (which do not correlate with the Book of Mormon), hold to a "limited" geographical model.
"[O]nce you say there were other people here, you say: OK, where were the Nephites, and how many more people were here. We have all kinds of other DNA signatures to worry about all of a sudden. It may be that we never find any Hebrew DNA (whatever that looks like) in the New World. ... But if we do find some, that's fine; if we don't find some, that's fine too. There's no way that negative evidence on that hurts the Book of Mormon whatsoever once you believe in a limited geography. If you believe in a global geography, you're basically done, toasted, game over." -John Clark (Professor of Anthropology and Director of the New World Archeological Foundation, BYU)  Others hold to the traditional archaeological model of the Book of Mormon. This is either held as a traditional belief, or in appeal to the Mormon organization's historic, consistent stance (from the beginning for over 100 years) on the Native Americans as Lamanites. Also of interest to those who hold this position is an incident where a group with Smith came upon bones, which Smith prophetically declared to belong to a white Lamanite named Zelph who had fought a great battle for the Nephites.  
"...the visions of the past being opened to my understanding by the Spirit of the Almighty, I discovered the person whose skeleton was before us was a white Lamanite, a large, thick-set man, and a man of God. His name was Zelph ... who was known from the Hill Cumorah, or eastern sea to the Rocky mountains." (History of the Church, 1948 ed., II: 79-80) The introduction to the Book of Mormon, since 1981, describes the Lamanites as, "the principal ancestors of the American Indians." 
- "If belief in the Book of Mormon was a prerequisite to joining the Church, it was a belief in the book's divine origin rather than the doctrinal content of the book. The Book of Mormon taught nothing different from what early 19th-century religious seekers would have already been familiar with. The theology of the Book of Mormon was monotheistic. Early Mormon theology then would not have been unique in comparison to other beliefs of the day." -Kurt Widmer, Mormonism and the Nature of God: A Theological Evolution, 1830-1915
- "I would never tell anybody to try to prove the Book of Mormon is true through physical evidence, just because of the way metaphysics and epistemology work—it's not possible. And so, you have to get the testimony some other way, and then the evidence will become very clear." -John E. Clark (Professor of Anthropology and Director of the New World Archeological Foundation, BYU) 
- Translation or Divination? (Institute for Religious Research)
- Testing the Book of Mormon, by Robert M. Bowman, Jr.
- 1830 Book of Mormon Comparison Machine
Articles by Sandra Tanner -
Archaeology and the Book of Mormon (Wikipedia)
The Scientific Search for Nephite Remains, by Luke Wilson
What is LGT (Limited Geography Theory)? (Exmormon.org)
- Book of Mormon Geography, by John E. Clark (Mormon)
- Where Did Book of Mormon Events Take Place? (PDF) (Fairlds.org - Mormon)
- Book of Mormon (Online version - lds.org)