The Book of Mormon outlines God's dealing with the ancient inhabitants of the Americas, and contains the fullness of the gospel. It is a companion to the bible and adds additional truths to questions that are not overtly discussed or clearly presented in the Bible.
The Book of Mormon provides convincing evidence that Joseph Smith is a true prophet of God. Joseph Smith was directed by a heavenly messenger named Moroni to a hill where gold plates had been hidden for centuries. These gold plates were translated by the power of God through Joseph Smith and are now know as The Book of Mormon.
When a person is investigating the Church of Jesus Christ of Latter Day Saints, they are invited to read, ponder and pray about the Book of Mormon. This is because there is no other way to know whether the book is true, than to read it, ask God, and learn for yourself. Watch this 2 minute introduction to The Book of Mormon, then, if you're interested in your own copy to read, go here for your own free Book of Mormon.