This book of the law shall not depart out of your mouth, but you shall meditate on it day and night, that you may be careful to do according to all that is written in it; for then you shall make your way prosperous, and then you shall have good success.
Rings are reminders. Five gold rings are gifts given to somebody who needs a lot of reminding. The five books of the Law are also written as reminders. They remind us that we are made in the image of God, that we have fallen, that God aims to save us, and that we often work at cross-purposes with Him even when He wills our good since we are a "stiff-necked and rebellious people." They remind us that despite this unpleasant diagnosis, God still wills our good and is determined that we shall have it. The whole point of the command given by Joshua in today's verse is for Israel to remember. The paradox of the command is that Israel will discover through long centuries of attempts the deepest reminder of the book of the Law: namely, that it cannot be kept. It was to teach this all-important lesson that the law was given, so that when Christ came He would find those who recognized that the law could not save, but could only point us to our need of salvation and ready us for the coming of the Savior.