What did Jehoshaphat's officials carry as they taught throughout Judah?

Answer: The Book of the Law

In his third year Jehoshaphat sent officials and Levites through the towns of Judah to teach, taking the Book of the Law.

“They taught throughout Judah, taking with them the Book of the Law of the LORD. They went throughout the towns of Judah and taught the people.”— 2 Chronicles 17:9 (BSB)