And thus ended the account of , and Helaman his son, and also Shiblon, who was his son.The fact that Nephi divides his writings into two books is even more puzzling when you realize that 2 Nephi just continues where 1 Nephi left off. It doesn't start out with a major shift of any kind. At the end of 1 Nephi, Nephi teaches his brothers by reading and commenting on the brass plates. This is what the beginning of 2 Nephi says:
And now it came to pass that after I, Nephi, had made an end of teaching my brethren...This is a very natural chapter break, but why a new book? I have seen some talk before of the entire book of 1 Nephi possibly being a chiasmus. So my thought was that perhaps Nephi was making this division to mark that structure and keep it intact by a separate book. Doing a search I found this site that seems to be the best outline of the chiastic structure of 1 Nephi that I have seen: http://wallacebking.net/ON_MY_MIND/Chiastic_Nephi.html. One can always ask if a chiasmus is intentional or forced because you specifically looked for it and tend to find what you're looking for. But I think the structure as outlined here compelling. It's a very beautiful chiasm with Christ as the center. I also found that this webpage proposes the exact same reason for the making of two books that I had thought about. In fact it is the only reasonable explanation of this question that I have seen so far.
Another thought to go along with this: In 2 Nephi 5:30-31, Nephi is asked to make the small plates. We know that 1 Nephi as we have it in the Book of Mormon is a translation from these small plates. This means that everything Nephi writes in 1 Nephi is his reflections on what has already happened. If he would write his history as the events occured, it would certainly be very unlikely to end up in chiastic form. But in this case, Nephi had time to plan, outline and draft a text before engraving it on plates.