I have found. — Literally, the book of the Torah have I found. The definite form of the expression proves that what the high priest found was something already known; it was not a book, but the book of the Law. How little the critics are agreed as to the precise character and contents of the book in question is well shown by Thenius: “Neither the entire then existing Scripture (Sebastian Schmidt), nor the Pentateuch (Josephus, Clericus, Von Lengerke, Keil, Bähr,) nor the ordered collection of Mosaic laws contained in Exodus, Leviticus, and Numbers (Bertheau), nor the book of Exodus (Gramberg), nor the book of Deuteronomy (Reuss, Ewald, Hitzig) is to be understood by this expression. All these must have been brought into their present shape at a later time. What is meant is a collection of the statutes and ordinances of Moses, which has been worked up (verarbeitet) in the Pentateuch, and especially in Deuteronomy. This work is referred to by Jeremiah (Jeremiah 11:1-17),and was called “The Book of the Covenant” (2 Kings 23:2). According to 2 Chronicles 17:9 it already existed in the time of Jehoshaphat (comp. 2 Kings 11:12, “the Testimony”); was probably preserved in the Ark (Deuteronomy 31:26), along with which in the reign of Manasseh it was put on one side. When after half a century of disuse it was found again by the high priest in going through the chambers of the Temple with a view to the intended repairs, in the Ark which, though cast aside, was still kept in the Temple, it appeared like something new, because it had been wholly forgotten (for a time), so that Shaphan could say: ‘Hilkiah has given me a book’ (2 Kings 22:10).” (See also the Notes on 2 Chronicles 34:14.)
And he read it. — Thenius thinks that this indicates that the book was of no great size, as Shaphan made his report to the king immediately after the execution of his commission (2 Kings 22:9). But neither does 2 Kings 22:9 say immediately, nor does this phrase necessarily mean that Shaphan read the book through.