Albert Barnes 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)

There is no reasonable doubt that this “book of the law” was the Pentateuch, in nearly the same form we have it today. Copies of the whole Law were undoubtedly scarce, and for this reason, the commission Jehoshaphat sent made sure to carry a copy with them.