‘ And Joshua wrote these words in the book of the law of God, and he took a great stone and set it up there under an oak, that was by the sanctuary of YHWH.'
The book of the Law of God is probably the same as the book of the Law and the book of the law of Moses (Joshua 8:31; Joshua 8:34 compare Exodus 24:4; Deuteronomy 31:9; Deuteronomy 31:24). It would thus include at least Exodus 20-24 and the basic Deuteronomy. It was kept beside the Ark of the Covenant in the Tabernacle (Deuteronomy 31:26). It is significant that Joshua recorded this solemn covenant in that book. He saw his covenant as part of the law of God. It demonstrates that it was the custom to record such covenants in writing, and we can compare how the main part of Genesis is made up of covenants set in their historical background, suggesting that they too had been so recorded.
“And he took a great stone.” Stones or pillars were regularly set up as memorials of covenants (compare Exodus 24:4; Genesis 28:18) and as a witness to the covenant. It is possible that he wrote the words of the covenant on the stone (compare Joshua 8:32; Deuteronomy 27:2-3).
“Set it up there under an oak that was in (or ‘by”) the sanctuary of YHWH.' Oaks were seen as having special significance. They were favourite trees under which to sit, presumably for shelter from the sun (1 Kings 13:14) or to bury the dead (Genesis 35:8; 1 Chronicles 10:12), possibly because they were landmarks (1 Samuel 10:3). Abram received a revelation under the oak of Moreh at Shechem (Genesis 12:6-7). Jacob buried the foreign gods of his household under an oak connected with Shechem (Genesis 35:4). But this oak was by (or even possibly ‘in') the sanctuary of God. It is doubtful if it was Abram's oak or Jacob's oak or even the oak of Meonenim (‘the diviner's oak' - Judges 9:37), for the sanctuary of God was probably that established on Mount Ebal (Joshua 8:30). It was simply a mark of where the stone was placed (it was not called on as a witness or referred to in any special way. It was only a marker).