Albert Barnes Commentary Exodus 17:14

"And Jehovah said unto Moses, Write this for a memorial in a book, and rehearse it in the ears of Joshua: that I will utterly blot out the remembrance of Amalek from under heaven." — Exodus 17:14 (ASV)

In a book - in the book, that is, the book which contained the history of God’s dealings with His people. Moses was further instructed to impress the command especially on the mind of Joshua, as the leader to whom the first step toward its accomplishment would be entrusted in the conquest of Canaan. The work was not actually completed until the reign of Hezekiah (1 Chronicles 4:43).