TL;DR The Book of Mormon

If you've always been curious about The Book of Mormon, sometimes called The Mormon Bible (though Mormons call it Another Testament of Christ), but never had time to read it, this article is for you! The Book of Mormon was published in 1830 by Joseph Smith, the founder of Mormonism, who claimed it was a translation from golden plates which he found hidden in the Hill Cumorah (in upper New York state), where he was directed to them by an angel. The book contains the story of a family of Jews (Father Lehi, sons Nephi, Laman, Lemuel) who leave Jerusalem before the Babylonian captivity and travel to America by way of a large ship, and wait for the promised appearance of Jesus Christ to their descendants.

The first part of The Book of Mormon is about the revelation Lehi receives in a dream to leave Jerusalem, and also about his vision of the "Tree of Life," which includes darkness and people in a wide and spacious building mocking the righteous people who are trying to make their way to the tree. Nephi, Lehi's third son, believes in his father's vision and the command to leave Jerusalem. He records the history of his brother's complaints and about their quest to return to Jerusalem according to God's commands, to get the records of their people from Laban, whom Nephi eventually has to kill (using a sword to cut off his head) because it is "better for one man to die than a whole nation to dwindle in unbelief." Nephi is chosen to receive revelation to build a ship which takes the family to America, where they then separate into warring two factions (Nephites and Lamanites).

An oft quoted scripture, Nephi 3:7:

And it came to pass that I, Nephi said unto my father: I will go and do the things which the Lord hath commanded, for I know that the Lord giveth no commandments unto the children of men, save he shall prepare a way for them that they may accomplish the thing which he commandeth them.

The second part of The Book of Mormon is about the wars between the Nephites and Lamanites and about the missionary efforts to the Lamanites many generations later. A favorite story in this section is about Ammon, who goes to King Lamoni and offers to become his servant. He tends the flocks and cuts off the arms of a group of bandits who try to steal them. Impressed with his courage and strength, King Lamoni offers to marry Ammon to one of his daughters and give him part of the kingdom, which Ammon refuses because he only wants to teach about the coming of Jesus Christ. Eventually, King Lamoni is converted and so are many of the Lamanites, including a group who put down their arms and refuse to kill ever again, even when they are slaughtered by other Lamanites for this betrayal of their heritage. The Nephites take them in and protect them, and in the next generation, their sons become great warriors at a young age and help the Nephites in turn.

An oft quoted scripture, Alma 56: 47-48:

Yea, they had been taught by their mothers, that if they did not doubt, God would deliver them. And they rehearsed unto me the words of their mothers, saying: We do not doubt our mothers knew it.

The third part of The Book of Mormon is about Jesus Christ's visit to this group in the Americas. The prophet Samuel the Lamanite warns the people that Christ will come on a specific date and is nearly killed for his troubles. When the year of the prophecy comes, the people who believe in it face execution, but are saved when the signs come true (a day and a night and a day without darkness). Thirty years later, after signs of Christ's death and numerous earthquakes and other destructions that level wicked cities, Christ Himself appears to the gathered righteous and delivers to them many of the same words (especially the Sermon on the Mount) that He delivered to His followers in Jerusalem. He also tells them that they are some of the "other sheep which are not of this fold," and hints that there may be others He will go to visit and preach to.

An oft quoted scripture, 4 Nephi 1: 2-3

The people were all converted unto the Lord, upon all the face of the land, both Nephites and Lamanites, and there were no contentions and disputations among them, and every man did deal justly one with another. And they had all things common among them; therefore there were not rich and poor, bond and free, but they were all made free, and partakers of the heavenly gift.

The fourth part of The Book of Mormon is about what happens after Christ's appearance in America. As has been true for much of the history of this people, while they are humble and willing to share their wealth with each other equally, they are blessed by God with material prosperity, but as soon as they reject the gospel of Christ and become greedy and jealous, gangs of thieves (called Gadianton robbers) take control of their government and cause wars, and ultimately all of the Nephites are destroyed and the prophet Mormon (for whom the book is named) dies and his son Moroni is the last living Nephite, hunted by the Lamanites until he hides the golden plates in the hill Cumorah for Joseph Smith to find them centuries later.

An oft quoted scripture, Mormon 8:16-17:

And blessed be he that shall bring this thing (the Book of Mormon) to light; for it shall be brought out of darkness unto light, according to the word of God; yea, it shall be brought out of the earth, and it shall shine forth out of darkness, and come unto the knowledge of the people; and it shall be done by the power of God. And if there be faults they be the faults of a man.

(There is also a section of The Book of Mormon about another group of people who come to the Americas right after the time of the Tower of Babel, who become wicked and eventually destroy themselves so completely that only one man remains to tell about their history.)

The Book of Mormon is additional scripture because Mormons believe that God continues to give revelation to His children in modern times and because Mormons believe that there are many groups which must have had contact with God and His prophets to whom Christ referred to when he said "Other sheep have I which are not of this fold."