Matthew Poole and continuators on Deuteronomy 31:26

26 Take this book of the law, and put it in the side of the ark of the covenant of the LORD your God, that it may be there for a witness against thee :

(i. e.) In the outside, in a little chest fixed to it, for nothing but the Tables of stone were contained in the Ark, 1 King. 8. 9. Here it was kept for greater security and reverence.

(i. e.) Against thy people, to whom he here turns his speech that they might be more affected with it.