by A critical subject that I think all missionaries should read is the King Follet Sermon. The sermon given by Joseph Smith explains premortal life, the nature of God, and exaltation simply and profoundly. It is the best resource that I have read on these subjects. Its saddening that there are missionaries who do not understand these concepts until after their mission. They think they know all about it just by hearing about it once or twice in church, but as missionaries, I think everyone should read this to have a better understanding of this doctrine as it came out of the mouth of Joseph Smith the prophet.
https://www.churchofjesuschrist.org/study/ensign/1971/04/the-king-follett-sermon?lang=eng
https://www.churchofjesuschrist.org/study/ensign/1971/05/the-king-follett-sermon?lang=eng
