Even though we have several books of the Bible they are all united. So if one book says “do not add” it means “do not add” to Divine Revelation and not just “do not add” to that one book.
The Canon was set a long time ago and we cannot add or subtract from it:
The Council of Hippo in 393 AD states:
[i:mwwai9wx]”[It has been decided] that [b:mwwai9wx]besides the canonical Scriptures nothing be read in church under the name of divine Scripture[/b:mwwai9wx]. But the canonical Scriptures are as follows: Genesis, Exodus, Leviticus, Numbers, Deuteronomy, Joshua the Son of Nun, Judges, Ruth, the Kings, four books, the Chronicles, two books, Job, the Psalter, the five books of Solomon, the twelve books of the Prophets, Isaiah, Jeremiah, Daniel, Ezekiel, Tobit, Judith, Esther, Ezra, two books, Maccabees, two books . . .” (canon 36 [A.D. 393]).[/i:mwwai9wx]
Second Council of Nicaea in 787 AD states:
[i:mwwai9wx]Ancient Epitome: We gladly embrace the Divine Canons, viz.: those of the Holy Apostles, of the Six Ecumenical Synods, as also of the local synods and of our Holy Fathers, as inspired by one and the same Holy Spirit. Whom they anathematize we also anathematize; whom they depose, we depose; whom they cut off, we cut off; and whom they subject to penalties, we also so subject.[/i:mwwai9wx]