Short answer: The Book of Mormon.
Have you ever wondered why Mormons have that book anyway? Why do they read it? Where did it come from? What's in it? Why would we need another book from God? Doesn't the Bible have everything we need?
Lemme 'splain. A few years after the incredible vision that Joseph Smith had on that spring morning in 1820, he was told by an angel (named Moroni) that "there was a book deposited, written upon gold plates, giving an account of the former inhabitants of this continent, and the source from whence they sprang. He also said that the fullness of the everlasting Gospel was contained in it, as delivered by the Savior to the ancient inhabitants."
So that means that Joseph was told where this book had been hidden by Moroni himself 1400 years ago, and that this book was the religious history of the people who had once inhabited the ancient Americas. Naturally, Joseph was excited to go and get this book of books. When the time was right, Joseph was told to go retrieve the plates, keep them safe, and to go about translating them.
So the question is this: How does an uneducated farmboy, who hasn't even reached his twenty-second year go a translate a book that is written in a language that had long-since died? Answer: With God's help. Joseph was given the ability to translate the record word for word from the original language, which was a type of reformed Egyptian. Obviously, the task would have been impossible to accomplish unless he had been inspired by the Almighty, and given the necessary tools to do so. With the help of several scribes, Joseph translated the book in a very short period of time and had the book published as The Book of Mormon (named after one of the principle characters in the book).
So what does the book talk about? Lemme sum up: Around 600 B.C., a family leaves Jerusalem because they were warned by God that the city would be destroyed (and it later was). They go to the sea, build a ship (God tells them how) and they cross the ocean, landing somewhere in the Americas (probably Central/South America). When they get there, the family splits up, creating two different nations or civilizations, called the Nephites and the Lamanites. The rest of the book is a history of those two peoples, focusing heavily on the teachings, prophecies, writings, sermons, and actions of their prophets. The book itself is a summary that was created by a prophet/historian/warrior named Mormon (that's why it has his name), but the concluding chapters were written by his son, Moroni (the same one who came to Joseph Smith as an angel and told him where the book was. He knew where it was because he put it there). By the end of the book, only the Lamanites remain, and they become the principle ancestors of the the American natives who were living here when the Europeans came over a thousand years later.
Okay, so that's the brief summary. Next question: why the heck do we need to know about all of that? Well, God knew that many would doubt the veracity of Joseph's claim that he had been called by God to restore the primitive church of Jesus Christ. Even more would doubt that he had actually seen the Father and the Son face to face. So He brought forth this book as spiritual evidence that Joseph Smith was a prophet and His chosen servant. One can know that the message of the Restoration is true if he will read the Book of Mormon and pray to know that it is true. As the introduction of the book says, "We invite all men everywhere to read the Book of Mormon, to ponder in their hearts the message it contains, and then to ask God, the Eternal Father, in the name of Christ if the book is true. Those who pursue this course and ask in faith will gain a testimony of its truth and divinity by the power of the Holy Ghost. (See Moroni 10:3-5.) Those who gain this divine witness from the Holy Spirit will also come to know by the same power that Jesus Christ is the Savior of the world, that Joseph Smith is his revelator and prophet in these last days, and that The Church of Jesus Christ of Latter-day Saints is the Lord’s kingdom once again established on the earth, preparatory to the second coming of the Messiah."
And that's why it is here. To show the entire world that God still talks to His children through ordained prophets, that the fullness of the Gospel is once again available to all mankind, and that the Church of Jesus Christ has been reestablished in its pure form in the latter days. That is our message. That is why Mormons try so hard to talk about the Book of Mormon, because it is the only way to know if this message if true. And if the book is true, everything else (Joseph Smith, the priesthood, ordinances such as baptism, etc.) would also be true. So my invitation to everyone who may read my blog is to go and read the Book of Mormon, and then ask God in prayer if it is true. He will answer you. He answered me.
And here is the testimony of one of the modern apostles of Jesus Christ as he testifies of the Book of Mormon. Enjoy!