This book of the law shall not depart from your mouth, but you shall meditate on it day and night, that you may observe to do according to all that is written in it; for then you shall make your way prosperous, and then you shall have good success. … Be strong and courageous. (Joshua 1:8-9 WEB, emphasis mine)
Prosperous in what? Good success in what? According to the context of Joshua:
Chapter One of Joshua comes after the five books of the Pentateuch that tell us about life with God and for God, and before the rest of Joshua which is concerned with taking possession of the promised land. The “promised land” could have a double meaning, such as being a symbol of a possession that lasts forever, that sustains and satisfies us forever, and a right relationship with God who lives and reigns forever.
Be strong and courageous doing all that is in God’s word, and you will have good success in sharing life with God and living for Him, in being transformed into His loving and glorious likeness, and in doing all that He has in His heart for you to do.