Num. 21:
14 Wherefore it is said in the book of the wars of the LORD, What he did in the Red sea, and in the brooks of Arnon,
Josh. 10:
13 And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day.
Ex. 24:
7 And he took the book of the covenant, and read in the audience of the people: and they said, All that the LORD hath said will we do, and be obedient
Yes, some unknown books are mentioned in the bible, I agree. But the Lord, to our knowledge, has not preserved them. Perhaps they will be found, or perhaps they were not inspired. Thanks.
<< Ex. 24:
7 And he took the book of the covenant, and read in the audience of the people: and they said, All that the LORD hath said will we do, and be obedient >>
Those first two books mentioned in scripture are never SAID to be in scripture. They were just books.
And this third one I requoted above -- that is talking about the Torah -- or at least the part that had been written by then -- primarily the ten commandments and a few dozen other laws. The "covenant" was the covenant at Sinai, where the people said they would obey everything God told them, and Moses had the priests sprinkle blood on the people and on the book, to ratify the covenant.
This is what is being referred to here -- not some lost book.