One of the books in the Book of Mormon
. It contains the crowning point of the book of Mormon, which is the personal ministry of Jesus Christ in ancient America. Missionaries often refer their investigators to this part of the book. You can read it here
for free online at The Church of Jesus Christ of Latter-day Saint's (informally the Mormon Church) official website in 3 Nephi 11-26.
This book is named after Nephi, the son of Nephi, who was the son of Helaman, the third prophet named Nephi in the Book of Mormon.