Jamieson, Fausset and Brown on Deuteronomy 17:18-20

18-20. he shall write him a copy of this law in a book--The original scroll of the ancient Scriptures was deposited in the sanctuary under the strict custody of the priests (see on De 31:26; 2Ki 22:8). Each monarch, on his accession, was to be furnished with a true and faithful copy, which he was to keep constantly beside him, and daily peruse it, that his character and sentiments being cast into its sanctifying mould, he might discharge his royal functions in the spirit of faith and piety, of humility and a love or righteousness.