3 Ne. 17:4; Companion to Your Study of the Book of Mormon, Ludlow, p. 271; Conference Report, Apr. 1916, p. 130; Conference Report, Oct. 1916, p. 76; refer in this text to 3 Ne. 21:28-29
“The splendid Book of Mormon advises that a third scriptural witness is yet to come from the lost tribes (see 2 Nephi 29:12-14)… . We do not know when and how this will occur, but we are safe in assuming that the third book will have the same fundamental focus as the Book of Mormon—‘that … their seed [too] … may be brought to a knowledge of me, their Redeemer’ (3 Nephi 16:4). If there is a title page in that third set of sacred records, it is not likely to differ in purpose from the title page in the Book of Mormon, except for its focus on still other peoples who likewise received a personal visit from the resurrected Jesus (see 3 Nephi 15:20-24; 16:1-4).” (Neal A. Maxwell, First Nephi, The Doctrinal Foundation, BYU Religious Studies Center, p. 15)
“Lost books are among the treasures yet to come forth. Over twenty of these are mentioned in the existing scriptures. Perhaps most startling and voluminous will be the records of the lost tribes of Israel (see 2 Nephi 29:13). We would not even know of the impending third witness for Christ except through the precious Book of Mormon, the second witness for Christ! This third set of sacred records will thus complete a triad of truth. Then, just as the Perfect Shepherd has said, ‘My word also shall be gathered in one’ (v. 14). There will be ‘one fold and one shepherd’ (1 Nephi 22:25) in a welding together of all the Christian dispensations of human history (see D&C 128:18).” (Neal A. Maxwell, Ensign, Nov. 1986, p. 52)