The first book mentioned has the information needed to condemn every person appearing at the bar of God’s court (Habakkuk 1:13). Yet John’s readers need to know that God wants all relevant information to be present in his court as the trial commences, including the extra information contained in the other book.
12 And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.