“he wrote upon the stones a copy.” Commanded in Deuteronomy 27:46. A very public and permanent copy of the Law. This is almost certainly not the entire Tanack from Genesis through Deuteronomy, but a representative of the most important laws. This could include the Ten Commandments and the blessings and curses from Deuteronomy 27-28.
“the law of Moses.” The Hebrew is “the torah of Moses,” where “torah” is much more than “law.” The torah involves instruction in many different ways (see commentary on Prov. 1:8).
“which he had written in the presence.” That is, which Moses had written in the presence of the children of Israel. The prepositional phrase, “in the presence of the children of Israel” can also be placed after the words “he wrote” at the beginning of the sentence which would change the meaning of the verse, but in the Hebrew text the prepositional phrase comes at the end of the verse and it makes sense there.