Jamieson, Fausset and Brown on Deuteronomy 31:26

26. Take this book of the law, and put it in the side of the ark--The second copy of the law (see on De 31:9) was deposited for greater security and reverence in a little chest beside the ark of the covenant, for there was nothing contained within it but the tables of stone (1Ki 8:9). Others think it was put within the ark, it being certain, from the testimony of Paul (He 9:4), that there were once other things inside the ark, and that this was the copy found in the time of Josiah (2Ki 22:8).