Exodus 31:18 And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.
That is not referring to a "Book in the Bible".
Those "tables of stone" contained the "Ten Commandments", which are quoted in a number of Books in the Bible (including the Book of Exodus itself - Chapter 20, the Book of Deuteronomy - Chapter 5, and partially in the Book of Matthew - Chapter 19, and in the Book of Romans - Chapter 13), but every single one of those Books (including "Exodus", which you quoted from) were written by imperfect, weak, frail human beings who God used as His chosen instruments to communicate for Him.