The Jewish custom was for the first-born son to receive the largest inheritance, with some notable exceptions: Ephraim was chosen over Manasseh (Genesis 48:14, 20, 26), Isaac over Ishmael (Genesis 17:19-21 [25-28]), Jacob over Esau (Genesis 27:19:41), and Joseph over Reuben (1 Chronicles 5:1-2). [Zarahemla Research Foundation, Study Book of Mormon, p. 7]
“Inasmuch As Thou Shalt Keep My Commandments, Thou Shalt Be Made a Ruler and a Teacher over Thy Brethren”
In the Lord's first recorded speech to Nephi, He explained the conditions surrounding some special promises to him:
1. Nephi would be led to a land of promise and would prosper (1 Nephi 2:20).
2. Nephi would be made a ruler over his brethren (1 Nephi 2:22).
3. Nephi would be made a teacher (minister) over his brethren (1 Nephi 2:22).
4. If Nephi's brethren rebelled against the Lord, they would be cursed with a sore curse (1 Nephi 2:23).
These promises all came to be dramatically fulfilled. Nephi's testament to the fulfillment of these promises might be a reason why the writings of Nephi were divided into two books (First Nephi and Second Nephi). Nephi could have very easily written just a single book, therefore he must have had some purpose behind his decision to write two books. [Alan C. Miner, Personal Notes] [For further discussion of this situation, see the commentary on 1 Nephi: Title "His Reign and Ministry"]