The term "Book of the Covenant" appears in two places in the Old Testament:
A document Moses read to the people of Israel at Mount Sinai (Exodus 24:7)
A document found in the temple by the priest Hilkiah. Hilkiah found the document while the temple was being repaired during King Josiah’s reign (2 Kings 23:2, 21; 2 Chronicles 34:30).
The Book of the Covenant at Mount Sinai
The word "covenant" refers to the covenant laws God made with the people of Israel during the time of Moses. In Hebrew, "book" can mean any written document. It includes texts written on clay, stone tablets, or parchment scrolls. Ancient covenants were often written down. The main problem in understanding the two references to "book of the Covenant" is figuring out what exactly was in these documents.
The book Moses read at Mount Sinai may have referred to either the Ten Commandments or to the whole section of Exodus 20–23, without the narrative parts. When the people heard it, they responded, "All that the Lord has spoken we will do." This shows the document contained laws or rules, but we cannot know exactly what was in it.
The Bible says that Moses wrote this book (Exodus 24:4). This does not mean it could not include the Ten Commandments, even though the Bible clearly states God wrote those commands (32:15–16). It is possible that Moses also wrote down the Ten Commandments at an earlier stage (Exodus 19:25; 20:1).
Josiah's Book of the Covenant
The contents of the "book of the covenant" that King Josiah read to the people of Judah are not clearly known. Some scholars have tried to figure out what was in it by looking at the reforms Josiah made. They think these reforms match the teachings in the book of Deuteronomy.
However, this approach has several problems:
Some of Josiah's reforms are not mentioned anywhere in the Law. For example, he burned the chariots of the sun (2 Kings 23:11). This suggests Josiah may have been making his own interpretations based on the Law. This makes it hard to know how much of his reform came directly from the Book of the Covenant and how much came from his own understanding.
The account in 2 Chronicles 34:30–33 shows that much of the reform happened before the discovery of the Book of the Covenant.
On the other hand, 2 Kings clearly states that some of Josiah's reforms were based on the Book of the Covenant. The book must have contained instructions about the Passover celebration (2 Kings 23:21). It probably also had rules about mediums, wizards, and other idol worship practices, unless Josiah made these reforms based on his own understanding of the text.
Also, the book contained warnings that God would bring destruction if the people did not follow God's words (2 Kings 22:16–19). This suggests that Josiah's "Book of the Covenant" was larger than Exodus 21–23. In the older book, the Passover is only mentioned as the Feast of Unleavened Bread (Exodus 23:15). Exodus 22:18 might possibly be the basis for Josiah’s action against wizards. But in Exodus 21–23, there is no warning of judgment for disobedience strong enough to explain the wording in 2 Kings 22:16–19. The closest thing to it is Exodus 23:33.
Finally, Josiah’s Book of the Covenant is also called the Book of the Law (2 Kings 22:8). This suggests that many other references to the Book of the Law throughout the Old Testament might also be referring to the Book of the Covenant.