During the time of Moses and Joshua in the Old Testament the Book of the Law was the only connection the Hebrews had to God.  They didn’t know him outside what was revealed to them in the law written by Moses. 

But we have Jesus now and He has fulfilled many things.  One of the things that he is described as is ‘the Word of God’, the law and ordinances are fulfilled in him. He personally has replaced the book of the law in the lives of people.

John 1:1-3 In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God.

1 John 1:1 What was from the beginning what we have heard, what we have seen with our eyes, what we have looked at and touched with our hands, concerning the Word of life 2 and the life was manifested and we have seen and testify and proclaim to you the eternal life, which was with the father and was manifested to us.

Revelation 19:13 He is clothed with a robe dipped in blood, and His name is called The Word of God.

Jesus Christ also told the Pharisees that Moses and the law and the prophets “were about me”.  Jesus Christ is the book of the law, now it is him that we are to seek daily, we our told now to fix our eyes on him (Heb 12:1), not the old letter but on him, for in him is transformation, in him is the fullness of all things.

Hebrews 12:2 fixing our eyes on Jesus, the author and perfecter of our faith…

And he replaces the law, the law is fulfilled in him therefore it is theologically acceptable and even appropriate to replace references in scripture to ‘the book of the Law’ and the Word with ‘Jesus Christ’ himself.

One such passage that I have always liked is Joshua 1:8 This Book of the Law shall not depart from your mouth, but you shall meditate in it day and night, that you may observe to do according to all that is written in it. For then you will make your way prosperous and then you will have good success. 9 Have I not commanded you? Be strong and of good courage; do not be afraid nor be dismayed, for the Lord your God is with you wherever you go.”

Now see how this passage reads if we replace the book of the law with the Lord Jesus Christ.

Joshua 1:8 (in fullness) The Lord Jesus Christ shall not depart from your mouth, but you shall meditate on Him day and night, that He may live his life through you. For then you will make your way prosperous and then you will have good success. 9 Have I not commanded you? Be strong and of good courage; do not be afraid nor be dismayed, for the Lord Jesus Christ is with you wherever you go.”

Jesus Christ shall not depart from our mouths.

Meditate on the Lord Jesus Christ day and night, not just bible verses anymore, which educate the mind but personally the Lord Jesus Christ.