The Book of Jacob is the third book of the Book of Mormon. Its full title is The Book of Jacob: The Brother of Nephi. According to the text, it was written by the ancient prophet Jacob. The purpose of the book, in his own words, is to persuade all men to "come unto Christ" (Jacob 1:7). While this book contains some history of the Nephites, including the death of Nephi, it is mainly a record of Jacob's preachings to his people. Chapter 5 contains The Parable of the Olive Tree, a lengthy allegory of the scattering and gathering of Israel, comparing the Israelites and gentiles to tame and wild olive trees, respectively.
In 545 BCE it was the turn for Nephi's brother Jacob to write on the plates, because Nephi had grown old and he saw that he was soon to die. The people were so happy with King Nephi that they wanted to call the next king Nephi II, and then Nephi III, and so on. Jacob writes that all the Jacobites, Josephites, and Zoramites were called Nephites together with the actual Nephites, and opposed to them were all the Lemuelites and Ishmaelites who were called Lamanites together with the actual Lamanites.
And the Nephites began to be lifted up in pride because they had been so blessed by God, and this led them to do wicked things. But Jacob and his brother Joseph tried as hard as they could to teach the people not to do wicked things, because if they didn't, they (Jacob and Joseph) would receive all the blame for their wickedness on the last day. "...otherwise their blood would come upon our garments, and we would not be found spotless at the last day."
Jacob went to the temple to teach the Nephites. This temple was a copy of the one Solomon made in Jerusalem. Jacob said the people had begun to search for gold and silver. And some of them were better searchers than others, so they enhanced their apparel to show off that they were more ambitious and competent than their brothers. Jacob said, "Think of your brethren like unto yourselves, and be familiar with all and free with your substance, that they may be rich like unto you."
After that Jacob preached against the practice of polygamy: "Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none."