(Church history from official sources)

"In fact, the language in the sections of the Book of Mormon that correspond to parts of the Bible is quite regularly selected by Joseph Smith, rather than obtained through independent translation. For instance, there are over 400 verses in which the Nephite prophets quote from Isaiah, and half of these appear precisely as the King James version renders them. Summarizing the view taken by Latter-day Saint scholars on this point, Daniel H. Ludlow emphasizes the inherent variety of independent translation and concludes: “There appears to be only one answer to explain the word-for-word similarities between the verses of Isaiah in the Bible and the same verses in the Book of Mormon.” That is simply that Joseph Smith must have opened Isaiah and tested each mentioned verse by the Spirit: “If his translation was essentially the same as that of the King James version, he apparently quoted the verse from the Bible."

https://www.lds.org/ensign/1977/09/by-the-gift-and-power-of-god?lang=eng