The Book of Mormon (Another Testament of Jesus Christ) contains many linguistic similarities to the King James Bible. In some instances, entire passages of scripture are duplicated in the Book of Mormon. At other times the source is acknowledged as in the book of Second Nephi, where 18 chapters of the book of Isaiah are quoted. There are some 478 verses in the Book of Mormon which are quoted in some form or other from the book of Isaiah. Of these verses, one Mormon scholar notes that 201 of them match the King James version of the quote and another 207 show variations. In addition, 58 quotes from Isaiah found in the Book of Mormon are paraphrased versions of those found in the King James Bible. 
The Existence of Biblical passages
The existence of biblical passages in the Book of Mormon is explained in the text as being the result of Lehi’s family bringing with them a set of “brass plates” from Jerusalem containing the writings of Moses, Isaiah, Jeremiah, and several prophets not mentioned in the Bible. Regarding this record, 1 Nephi 5:10-13 states: 
And after they had given thanks unto the God of Israel, my father, Lehi, took the records which were engraven upon the plates of brass, and he did search them from the beginning. And he beheld that they did contain the five books of Moses, which gave an account of the creation of the world, and also of Adam and Eve, who were our first parents; and also a record of the Jews from the beginning, even down to the commencement of the reign of Zedekiah, king of Judah; and also the prophecies of the holy prophets, from the beginning, even down to the commencement of the reign of Zedekiah; and also many prophecies which have been spoken by the mouth of Jeremiah.
Stephen R. Gibson, author of One Minute Answers to Anti-Mormon Questions stated:
Of the approximately 264 thousand words in the Book of Mormon, about 17 thousand are close parallels to the King James translation of the Bible. Most parallel verses occurred when Nephi quoted the Isaiah of the Old Testament using records on brass plates brought from Jerusalem. Other parallels occurred when the resurrected Savior repeated his Sermon on the Mount to the Nephites and then quoted Malachi at length. In both cases, we are told in the text that these are quotations of scriptures that had been recorded elsewhere.
Why would God render the Book of Mormon translation into KJV English?
As more than one LDS scholar has pointed out, the KJV English was the accepted scriptural language of Joseph Smith’s day. When Jesus, the Apostles, or even the angel Gabriel quote scriptures in the New Testament they do not quote from some ancient and perhaps original source. Instead they quote from the Septuagint—the Greek version of the Old Testament, which was the accepted Bible of New Testament readers. “When ‘holy men of God’ quote the scriptures,” notes Nibley, “it is always in the received standard version of the people they are addressing…” Likewise, the scriptural language of Joseph Smith’s day was King James English. Quite often when other ancient texts— such as the Dead Sea Scrolls—are translated into English, they—like the Book of Mormon—are rendered into King James English. One can hardly chide Joseph for doing the very same thing that modern scholars often do when translating ancient religious texts. 
Stephen R. Gibson further stated:
LDS scholars generally agree that in instances where the Book of Mormon parallels the Bible, Joseph Smith must have noted the parallels and used the King James Bible to guide him in his choice of words. If the Book of Mormon agreed with the Biblical text in meaning, he apparently utilized the Biblical text, italicized words and all. However, when the plates differed from the Biblical text, he followed the text on the plates.
Since the ultimate source of the teachings of the Book of Mormon is Christ, and since the ultimate source of the teachings of the Bible (both Old and New Testaments) is also Christ, it should surprise no one that there are many parallels between the Book of Mormon and Bible passages. While one cannot conclusively prove that Joseph Smith used a Kings James Bible as an aid in translating the parallel passages, that explanation is reasonable. (Stephen R. Gibson, One Minute Answers to Anti-Mormon Questions, pg. 21-22)