The holy - The word in the Hebrew is plural, agreeing, probably, with אלהים 'elohı̂ym understood (so in Proverbs 30:3). The knowledge of the Most Holy One stands as the counterpart to the fear of Yahweh.
Related Commentaries of Proverbs 9:10
Proverbs 9:10
10 The fear of the LORD is the beginning of wisdom: and the knowledge of the holy is understanding.