When talking to Protestants, I always make sure not to say "The Catholic canon has seven more books," but rather "The Protestant canon has seven fewer books." It actually makes a difference how you phrase it.
Also, it is a little tedious, but it is worth pointing out that the Protestant Bible didn't really exist before the Protestant Reformation. Until said reformation, there were two sets of Old Testament (in the West):
*The 46 books, in the following order: five Books of Moses, Joshua, Judges, Ruth, four Books of Kings (or two Books of Samuel and two Books of Kings), two Books of Chronicles, two Books of Ezra (Ezra and Nehemiah), Tobith, Judith, Esther, Job, the Book of Psalms, four Books of Solomon (Proverbs, Ecclesiastes, Song of Songs, Wisdom), Sirach, Isaiah, Jeremaiah, Lamentations, Baruch (with the Letter of Jeremiah), Ezekiel, Daniel, the Twelve Books of the Minor Prophets, and two Books of the Maccabees. This is how it is in the Septuagint and the Vulgate, and was used by the Christian West.
*The 24 books, in the following order: The Torah, that is the Five Books of Moses; Eight Books of the Prophets, these are Joshua, Judges, Samuel, Kings, Isaiah, Jeremiah, Ezekiel, and the Twelve Prophets; and Eleven Books of the Writings, these are Psalms, Proverbs, Job, Song og Songs, Ruth, Lamentations, Ecclesiastes, Esther, Daniel, Ezra (i.e. Ezra and Nehemiah) and Chronicles. This is how it is in the Hebrew Scriptures, and was used by the Jews.
So the modern Protestant canon is a chimera: it uses the Hebrew Scriptures only, but with the Greek/Latin division and order of the books. Does it matter? Probably not. But when the canon is given, the order is always fixed. So the Reformation wasn't a true return to Hebrew orthodoxy. It effectively made its own canon. Again, it's not a major point, but it is just one of many things showing that the Reformation wasn't a return "the the sources" (ad fontes), but its own thing with its own new ideas.
