I think about how many times I have started and restarted, and read and reread the Book of Mormon. And being a sequential studier that I am, I always start at. the. beginning.
Which would mean, technically, the book I have read most often in the Book of Mormon is, First Nephi.
And what would be the message of First (and Second) Nephi? Here is what Nephi teach by example:
Go and do the things the Lord commands, for He will provide the way (1 Nephi 3:7).
As they Lord lives and as we live, we will accomplish the Lord's commands (1 Nephi 3:15-16)
Be faithful in keeping in keeping the commandments of the Lord (1 Nephi 4:1-3)
Obey the voice of the Spirit (1 Nephi 4:17-18)
Be diligent in keeping the Lord's commandments (1 Nephi 4:34)
The Lord is able to do all things so let us be faithful to Him (1 Nephi 7:12-13)
The Lord know all things and has all power, and He will prepare a way for us to accomplish the fulfilling of His words (1 Nephi 9:6)
If we keep the commandments, God will nourish us, strengthen us and provide us with the means to obey His commandments (1 Nephi 17:3)
When we keep God's commandments, our obedience is an example to others. (1 Nephi 17:15)
If God commands us to do all things, we could do them (1 Nephi 17:50)
Being saved at the last day = being obedient + enduring to the end (1 Nephi 22:31)
Sometimes obedience requires doing things we do not completely understand (2 Nephi 5:30-31)
Being obedient doesn't necessarily mean it is easy (2 Nephi 33:11)
And the final words recorded by Nephi: "For thus hath the Lord commanded me, and I must obey." (2 Nephi 33:15)
Which brings me back to my first thought of why First and Second Nephi are where they are.
Think if the Book of Mormon began chronologically. That would put the Book of Ether at the very beginning of the Book of Mormon.
Which would mean the story I would read and reread, start and restart the most would be the one of the Jaredite people whom, because of wickedness and abomination and the refusal to repent, became destroyed and extinct.
But the Book of Ether is not what the Lord wants to be our beginning (over and over) lessons as we open "another testament of Jesus Christ."
He wants us to learn, through Nephi, that obedience is the first law of heaven.
And it is something I am learning and relearning everytime I begin at the very beginning of the Book of Mormon.