Here are some citations from the Book of Mormon and Doctrine and Covenants which teach that God is not a man, and that the Trinity is true.
1. "The Testimony of Three Witnesses" at the front of the Book of Mormon ends with the statement: "And the honor be to the Father, and to the Son, and to the Holy Ghost, which is one God. Amen."
2. 2 Nephi 11:7 makes the point that Jesus is God: "For if there be no Christ there be no God; and if there be no God we are not, for there could have been no creation. But there is a God, and he is Christ, and he cometh in the fulness of his own time."
3. 2 Nephi 26:12 says Jesus is the Eternal God: "And as I spake concerning the convincing of the Jews, that Jesus is the very Christ, it must needs be that the Gentiles be convinced also that Jesus is the Christ, the Eternal God."
4. Another Trinitarian statement is 2 Nephi 31:21: "And now, behold, my beloved brethren, this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end. Amen."
5. Here's a bit convoluted Trinitarian statement, which might be a good one to ask to be explained [comment - who is it that calls the father and son one person?]. Mosiah 15:1-5: "And now Abinadi said unto them: I would that ye should understand that God himself shall come down among the children of men, and shall redeem his people. And becasuse he dwelleth in flesh, he shall be called the Son of God, and having subjected the flesh to the will of the Father, being the Father and the Son - The Father, because he was conceived by the power of God; and the Son, because of the flesh; thus becoming the Father and the Son - And they are one God, yea, the very Eternal Father of heaven and of earth. And thus the flesh becoming subject to the Spirit, or the Son to the Father, being one God, suffereth temptation, but suffereth himself to be mocked, and scourged, and cast out, and disowned by his people."
6. Another one where they call the Son the Father: Mosiah 16:15: "Teach them that redemption cometh through Christ the Lord, who is the very Eternal Father."
7. Mormons preach multitudes of gods, even that they become one. But Alma 11:28 says, "Now Zeezrom said, Is there more than one God? And he answered, No."
8. Is God eternal or did He start as a man as LDS teach now? Is the son the father? Alma 11:38-39: "Now Zeezrom saith agian unto him: Is the Son of God the very Eternal Father? And Amulek said unto him: yea, he is the very Eternal Father of heaven and of earth, and all things which in them are; he is the beginning and the end, the first and the last."
9. Alma 11:44: "Now, this restoration shall come to all, both old and young, both bond and free, both male and female, both the wicked and the righteous; and even there shall not so much as a hair of their heads be lost; but every thing shall be restored to its perfect frame, as it is now, or in the body, and shall be brought and arraigned before the bar of Christ the Son, and God the Father, and the Holy Spirit, which is one Eternal God, to be judged according to their works, whether they be good or whether they be evil."
10. Is God man or spirit? Alma 18: 24: "And Ammon began to speak unto him with boldness, and said unto him: Believest thou that there is a God? And he answered, and said unto him: I do not know what that meaneth. And then Ammon said: Believest thou that there is a Great Spirit? And he said, Yea. And Ammon said: This is God."
11. Is God Spirit? Alma 22:9-11: "And the king said: Is God that Great Spirit that brought our fathers out of the land of Jerusalem? And Aaron said unto him: Yea, he is that Great Spirit, and he created all things both in heaven and in earth. Believest thou this? And he said: Yea, I believe that the Great Spirit created all things, and I desire that ye should tell me concerning all these things, and I will believe thy words."
12. Again, is God a spirit? Alma 31:15: "Holy, holy God; we believe that thou art God, and we believe that thou art holy, and that thou wast a spirit, and that thou art a spirit, and that thou wilt be a spirit forever."
13. 3 Nephi 11:27 and 36 has the trinity: "And after this manner shall ye baptize in my name: for behold, verily I say unto you, that the Father, and the Son, and the Holy Ghost are one.... And thus will the Father bear record of me, and the Holy Ghost will bear record unto him of the Father and me; for the Father, and I, and teh Holy Ghost are one."
14. D&C 68:8: "...baptizing in the name of the Father, and of the Son, and of the Holy Ghost."
15. D&C 93:3: "I am in the Father, and the Father in me, and the Father and I are one."
As you can easily see while reading these quotes, when Joseph Smith wrote the Book of Mormon he had a fairly orthodox theology. It was only later, especially after settling in Nauvoo, that Smith denied the Trinity and began teaching the theology of a plurality of gods and other bizarre doctrines.
This is just another example of the ever-changing teachings of the Mormon church.