There is so much information available about the history of Mormonism and the famous golden tablets elsewhere, that I will only deal very briefly with it here. In the strictest sense, the book of Mormon is not a Bible hoax.
Mormons believe that an angel showed Joseph Smith where to find the tablets, which he then translated. There is no claim that the text was written by an apostle or some other Biblical author. The Book of Mormon relates to the Bible in about the same way as the Quran relates to the Bible. It is the founding document of an entirely new religion, but draws heavily on material from one or more other religions. Most obvious, the language of the BoM imitates the King James version of the Bible.
The Mormons are thus not committing any forgery with their book; they openly admit that it is not Biblical but rather the work of their prophet Joseph Smith who claimed to have received the material in supernatural ways. The historic truth of what the BoM claims to have happened is quite another story...