Albert Barnes Commentary 2 Kings 22:8

"And Hilkiah the high priest said unto Shaphan the scribe, I have found the book of the law in the house of Jehovah. And Hilkiah delivered the book to Shaphan, and he read it." — 2 Kings 22:8 (ASV)

Some have concluded from this discovery either that no “book of the law” had ever existed before, arguing that the work now said to have been “found” was forged for the occasion by Hilkiah. An alternative view is that all knowledge of the old “book” had been lost, and that a work of unknown date and authorship, having been found at this time, was accepted as the Law of Moses on account of its contents and has thus come down to us under his name.

However, this is to read far more into the narrative than it naturally implies. If Hilkiah had been bold and wicked enough to forge the book, or foolish enough to hastily accept a composition he knew nothing about as the real “book of the law,” there were four ways to detect his error or fraud:

1. The Jewish Liturgies, which embodied large portions of the Law.
2. The memory of living people, which in many cases may have extended to the entire five books, as it does now with modern Samaritans.
3. Other copies, entire or fragmentary, existing among the more learned Jews or in the Schools of the Prophets.
4. Quotations from the Law in other works, especially in the psalmists and prophets, who refer to it on almost every page.

The copy of the Book of the Law found by Hilkiah was undoubtedly the one that Moses, in accordance with God’s command, had deposited by the side of the ark of the covenant, where it was ordinarily kept in the Holy of Holies. It had been lost or hidden away during the desecration of the temple by Manasseh but had not been removed from the temple building.