The book mentioned in 2 Nephi 28:2 is the Book of Mormon and is the same book Nephi referred to in 2 Nephi 26–27, 29. Nephi said “the book shall be of great worth” to us because of conditions in the last days. President Ezra Taft Benson (1899–1994) taught how we can recognize the great worth of the Book of Mormon:
“Each of the major writers of the Book of Mormon testified that he wrote for future generations. …
“If they saw our day and chose those things which would be of greatest worth to us, is not that how we should study the Book of Mormon? We should constantly ask ourselves, ‘Why did the Lord inspire Mormon (or Moroni or Alma) to include that in his record? What lesson can I learn from that to help me live in this day and age?’ …
“… In the Book of Mormon we find lessons for dealing with persecution and apostasy. We learn much about how to do missionary work. And more than anywhere else, we see in the Book of Mormon the dangers of materialism and setting our hearts on the things of the world. Can anyone doubt that this book was meant for us and that in it we find great power, great comfort, and great protection?” (in Conference Report, Oct. 1986, 5–6; or Ensign, Nov. 1986, 6–7).