Royal Skousen writes that First and Second Nephi were each originally just called "The Book of Nephi."
Royal Skousen, "The Original Text of the Book of Mormon and its Publication by Yale University Press," Interpreter: A Journal of Mormon Scripture 7 (2013), p. 70, accessed July 20, 2026
Next is the beginning of First Nephi. You’ll notice that the text just says “The Book of Nephi” – that’s because the manuscripts tell us that that was the original title for the book. There are, it turns out, four books of Nephi in the Book of Mormon. Later Oliver Cowdery and other editors added first, second, third, and fourth to the names of these individual books. The original text had no numbers for any of the books of Nephi. Since this is the way the original text read, we reproduce it that way in the Yale edition.