What was the “book of the law” that Hilkiah found in 2 Kings 22:8?

2 Chronicles 17:7 In the third year of his [Jehoshaphat] reign he sent his officials … 9 They taught throughout Judah, taking with them the Book of the Law of the Lord; they went around to all the towns of Judah and taught the people.

More than two centuries later in 2 Kings 22:8:

Hilkiah the high priest said to Shaphan the secretary, "I have found the Book of the Law in the temple of the LORD." He gave it to Shaphan, who read it.

What exactly was this book?

2 Kings 22:11 When the king heard the words of the book of the law, he tore his robes.

Was King Josiah surprised by the contents of the book?