The Book of Mormon is an ancient record of the people who inhabited the American continent. It was written by prophets who lived among the people and gives testimony of the Lord Jesus Christ as he visited this land after his resurrection. It has prophecies of America and how it would be inhabited by the gentiles from Europe. It reveals the evil works of secret combinations and tells of the future of America. It is a second witness to the Bible of the Savior and Redeemer of the world, even Jesus Christ. Moroni, was the last of the prophets who placed the record of gold plates, the Urim and Thummim, and the sword of Laban and sealed them up in a stone box and buried the stone box in the earth before he died. Later he was resurrected and was commissioned by the Lord to reveal the hiding place of the record to Joseph Smith. After 3 years, Joseph Smith was allowed to remove the contents of the box and he began translating the book via the Urim and Thummim (also known as seer stones). Later this same angel returned and took the plates, Urim and Thummim, and the sword of Laban.
Three witnesses were given the privilege of viewing and handling the plates, seeing the angel Moroni, and hearing the voice of God declare the truthfulness of the work unto them. You can read their testimony at this link:
Testimony of Three Witnesses
Later, eight more witnesses were given the privilege of seeing and handling the plates and have given their witness to the truthfulness of their existence:
Testimony of Eight Witnesses
The Book of Mormon comes with a promise given by the prophet Moroni as follows:
Moroni 10:2-5
2 And I seal up these records, after I have spoken a few words by way of exhortation unto you.
3 Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.
4 And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
5 And by the power of the Holy Ghost ye may know the truth of all things.