There is no reasonable doubt that this “book of the law” was the Pentateuch - nearly, if not quite, in the shape in which we now have it. Copies of the whole Law were, no doubt, scarce; and therefore Jehoshaphat’s commission took care to carry a copy with them.
Related Commentaries of 2 Chronicles 17:9
2 Chronicles 17:9
9 And they taught in Judah, and had the book of the law of the LORD with them, and went about throughout all the cities of Judah, and taught the people.