Solomon wasn’t the only wise man in the middle east (even if he was the wisest). This final group proverbs was written by Agur and King Lemuel, who wrote down his mother’s sayings. We don’t know who these people are now, but they would have been well known and respected when the book of proverbs was compiled.
In the middle of comparing himself (unfavourably) with God (see the overview), Agur stopped to contemplate the nature of God’s word:
Proverbs 30:5-6 Every word of God proves true;
he is a shield to those who take refuge in him.
Do not add to his words,
or else he will rebuke you, and you will be found a liar.