“ This book of the law shall not depart from your mouth, but you shall meditate in it day and night, that you may observe to do according to all that is written in it. For then you will make your way prosperous, and then you will have good success.”
The idea here is of learning by heart and then constant spoken repetition (they could not carry written books around in their pockets for the ‘books' were bulky and not portable). Day and night he was constantly to take the opportunity of repeating to himself the memorised word of God, and that with the aim of observing all that was in it. It is fine to rejoice in the promises of God, but we must also take careful note of the instructions of God.
The result will be success in what we do. Joshua's success would depend on his knowledge of and submission to the word of God.
“This book of the law.” See Deuteronomy 28:58; Deuteronomy 28:61; Deuteronomy 29:21; Deuteronomy 30:10. Reference is to ‘the book of the law' written down either by Moses or under his supervision. It may well be that Joshua had obtained the book from those responsible for watching over it for the very purpose of meditating on it. It was probably written on papyrus brought from Egypt, or possibly on leather. (He may have written it himself on Moses' instructions).
“Meditate in it day and night.” A thought taken up by the Psalmist in Psalms 1:2. If we would succeed with God we must meditate regularly on His word and ensure that we live out every word of it.