John Gill Commentary 2 Chronicles 17:9

"And they taught in Judah, having the book of the law of Jehovah with them; and they went about throughout all the cities of Judah, and taught among the people." — 2 Chronicles 17:9 (ASV)

And they taught in Judah
The people their duty to God and man:

and had the book of the law of the Lord with them ;
as the rule of their instruction, and the people's obedience; out of this they taught them, and by it confirmed what they taught, and enforced it with the divine authority:

and went about throughout all the cities of Judah ;
not only their own cities, the Levitical ones, but all others:

and taught the people ;
the law of the Lord, the fear and worship of God.