From a doctrinal perspective, these chapters are among the most important in the text, as the lesson manual explains very well. Nephi explains how anyone can come to know the truth about the doctrine of Christ, and what leads to eternal life:
"Wherefore, ye must press forward with a steadfastness in Christ, having a perfect brightness of hope, and a love of God and of all men. Wherefore, if ye shall press forward, feasting upon the word of Christ, and endure to the end, behold, thus saith the Father: Ye shall have eternal life."
These chapters don't directly contain any information relating to historicity; instead, they tell us how to find answers to our questions and how to discern truth. Nephi write, "For my soul delighteth in plainness; for after this manner doth the Lord God work among the children of men. For the Lord God giveth light unto the understanding; for he speaketh unto men according to their language, unto their understanding."
Here are some plain statements that I think are relevant to issues of Book of Mormon historicity:
1. Joseph Smith, the Wentworth letter: "The principal nation of the second race fell in battle towards the close of the fourth century. The remnant are the Indians that now inhabit this country."
You can find this clear, plain statement on lds.org here. You won't find it in the chapter on the Wentworth Letter in the lesson manual Teachings of the Presidents of the Church: Joseph Smith (here) because Mesoamerican scholars managed to get it deleted from the manual.
Joseph began the letter with this request: "As Mr. Bastow has taken the proper steps to obtain correct information, all that I shall ask at his hands is that he publish the account entire, ungarnished, and without misrepresentation." It turns out, Joseph didn't need to worry about Mr. Bastow. He needed to worry about the Curriculum Committee who deleted key passages from the lesson manual.
There has been an amazing effort to twist the plain meaning of Joseph's statement by using sophistry to claim he was referring to Central America. Just read materials published by FARMS, FairMormon, BMAF, etc.
2. The Lord called Oliver Cowdery to go on a mission in D&C 28:8 And now, behold, I say unto you that you shall go unto the Lamanites and preach my gospel unto them; and inasmuch as they receive thy teachings thou shalt cause my church to be established among them; and thou shalt have revelations, but write them not by way of commandment. 9 And now, behold, I say unto you that it is not revealed, and no man knoweth where the city Zion shall be built, but it shall be given hereafter. Behold, I say unto you that it shall be on the borders by the Lamanites.
3. The Lord called Peter Whitmer, Jr., to accompany Oliver in D&C 30:5 Behold, I say unto you, Peter, that you shall take your journey with your brother Oliver; for the time has come that it is expedient in me that you shall open your mouth to declare my gospel; therefore, fear not, but give heed unto the words and advice of your brother, which he shall give you. 6 And be you afflicted in all his afflictions, ever lifting up your heart unto me in prayer and faith, for his and your deliverance; for I have given unto him power to build up my church among the Lamanites;
4. The Lord then called Parley P. Pratt and Ziba Peterson to accompany Oliver and Peter in D&C 32:2 And that which I have appointed unto him is that he shall go with my servants, Oliver Cowdery and Peter Whitmer, Jun., into the wilderness among the Lamanites. 3 And Ziba Peterson also shall go with them; and I myself will go with them and be in their midst; and I am their advocate with the Father, and nothing shall prevail against them.
The revelations appear fairly "plain" to me, but Church scholars are hedging even on this. For example, the Joseph Smith Papers provide a Historical Introduction that casts these verses as a sort of quaint folk belief:
"In September and October 1830, a series of revelations directed Oliver Cowdery, Peter Whitmer Jr., Parley P. Pratt, and Ziba Peterson to serve a mission “among the Lamanites”—understood by them to be the American Indians."
I absolutely love the Joseph Smith papers, but I can't make sense of that explanation. It was not the missionaries who designated the American Indians as Lamanites--it was the Lord, through revelation.
Where did these missionaries go? They went to Native American Indian tribes in New York, Ohio, and Missouri (tribes that had been forced from their homelands around the Great Lakes). How could the Lord have made it any more plain that Lehi's descendants lived in the northeastern and midwestern United States?
Oliver, Peter, Parley and Ziba signed a mutual covenant regarding their mission. The editors of the Joseph Smith papers were referring to this when they made the comment about what these missionaries "understood," but it is plain that their understanding came directly from the Lord, not from some sort of common tradition of unknown origin that Joseph Smith simply adopted. Yet that is what the Mesoamerican advocates want people to believe. In fact, this idea that Oliver Cowdery naively thought the American Indians were Lamanites is also evident in the display placard at the new Church History museum.
Here's the covenant:
Manchester, Oct. 17, 1830.
I, Oliver Cowdery, being commanded of the Lord God, to go forth unto the Lamanites, to proclaim glad tidings of great joy unto them, by presenting unto them the fulness of the Gospel, of the only begotten son of God; and also, to rear up a pillar as a witness where the Temple of God shall be built, in the glorious New-Jerusalem; and having certain brothers with me, who are called of God to assist me, whose names are Parley P. Pratt, Peter Whitmer Jr. and Ziba Peterson, do therefore most solemnly covenant before God, that I will walk humbly before him, and do this business, and this glorious work according as he shall direct me by the Holy Ghost; ever praying for mine and their prosperity, and deliverance from bonds, and from imprisonments, and whatsoever may befal us, with all patience and faith.— Amen.
We, the undersigned, being called and commanded of the Lord God, to accompany our Brother Oliver Cowdery, to go to the Lamanites, and to assist in the above mentioned glorious work and business. We do, therefore, most solemnly covenant before God, that we will assist him faithfully in this thing, by giving heed unto all his words and advice, which is, or shall be given him by the spirit of truth, ever praying with all prayer and supplication, for our and his prosperity, and our deliverance from bonds, and imprisonments, and whatsoever may come upon us, with all patience and faith.—Amen.
Signed in presence of
JOSEPH SMITH, Jun.
P. P. PRATT,
PETER WHITMER Jr.