3 Nephi 26:9

But behold, there are many books and many records of every kind, and they have been kept chiefly by the Nephites.  Helaman 3:15

And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.

Doctrine and Covenants 8:1, 11

1  OLIVER Cowdery, verily, verily, I say unto you, that assuredly as the Lord liveth, who is your God and your Redeemer, even so surely shall you receive a knowledge of whatsoever things you shall ask in faith, with an honest heart, believing that you shall receive a knowledge concerning the engravings of old records, which are ancient, which contain those parts of my scripture of which has been spoken by the manifestation of my Spirit.

11  Ask that you may know the mysteries of God, and that you may translate and receive knowledge from all those ancient records which have been hid up, that are sacred; and according to your faith shall it be done unto you. 

Elder Neal A Maxwell – ‘God Will Yet Reveal’ 

‍https://www.lds.org/general-conference/1986/10/god-will-yet-reveal?lang=eng 

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 Ne. 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” (2 Ne. 29:14). There will be “one fold and one shepherd” (1 Ne. 22:25) in a welding together of all the Christian dispensations of human history (see D&C 128:18).

Whereas previous prophets were sometimes left to surmise—as Moroni supposed the Jews also had a record of the Creation from Adam on down (see Ether 1:3)—ours, instead, is a time of fulness, including “things which never have been revealed from the foundation of the world” (D&C 128:18). Moreover, “and the day cometh that the words of the book which were sealed shall be read upon the house tops; and they shall be read by the power of Christ; and all things shall be revealed unto the children of men which ever have been among the children of men, and which ever will be even unto the end of the earth” (2 Ne. 27:11; see also 2 Ne. 30:16, 18; Ether 4:7; D&C 101:32; D&C 121:28).

Thus, just as there will be many more Church members, families, wards, stakes, and temples—later on, there will also be many more nourishing and inspiring scriptures. However, we must first feast worthily upon that which we already have!