Q: Who are Mormons? Can you prove that Islam is true or their religon is wrong through their book?
A: Mormonism is an offshoot of Christianity. Its followers, called Mormons, believe that Jesus Christ appeared to one Joseph Smith of America in 1820 commanding him to restore his teachings. They also believe in the Trinity and general beliefs of Christians. However, mainstream Christianity do not accept them as Christians and believe the Mormons are a cult and not part of Christianity.
This prophet, who was also called Mormon, was supposed to have given them them a book called the Book of Mormon, which Mormons believe is an addition to the New Testament.
We have not yet had the opportunity to study their books and writings, so cannot at this stage provide the proof that you request.
In any case, from an Islamic perpsective, the Mormons are as false and misguided as the majority of Christians themselves.
There is no new prophet to come after the Pophet Muhammad (sallallahu alayhi wasallam). So this claim is totally false. As for their books, we will insha Allah study some of their writings and see if the proof you request is indeed available from their books.