John Gill Commentary Joshua 1:8

"This book of the law shall not depart out of thy mouth, but thou shalt meditate thereon day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success." — Joshua 1:8 (ASV)

This book of the law shall not depart out of your mouth
He was often to read it, frequently repeat it, and speak of it, to refresh his own memory with it, and the memory of those about him. Jarchi restrains it to the book of Deuteronomy now before him:

but you shall meditate therein day and night ;
whenever he had any leisure from the important business of his office, whether by day or night, see (Psalms 1:2) ;

that you may observe to do according to all that is written
therein ;
which frequent speaking of it, and constant meditation on it, would lead unto:

for then you shall make your way prosperous, and then you shall have
good success ;
in his wars with the Canaanites.