Angel Moroni Weather Vane from Mormon Temple in Nauvoo Illinois, 1846.

Reaching into the clouds from which the Son of Man was expected to one day descend, this weather vane atop the Nauvoo Temple serves a rhetorical purpose too.  Depicted is an angel trumpeting and carrying a book, a sign or signal of the end of the era.  As is implicit in the name of the Church of Jesus Christ of Latter-day Saints, the belief in the imminent second coming of Jesus was fundamental. 

Historical-Religious context - protestants protesting protestantism
Mormonism was founded in 1830.  It developed in a period later designated by scholars as the Second Great Awakening, and in a part of the US (NY state) that became known as the Burner Over District.  Both of these titles refer to the religious fervor that swept through this region during the time and spawned a number of new religious movements.  One such movement was the Millerites (1830's) who experienced the Great Disappointment in 1844 (Jesus didn't come back when expected).  From the Millerites came both the Jehovah's Witnesses (1870) and the Seventh Day Adventists (1863).  The time period also generated other Restoration movements such as the Stone-Campbell movement that spawned the Disciples of Christ, the Church of Christ and others. 

The Angel's journey, message, and trappings are all significant.  The trumpet has a deep cultural history and is attested through out biblical and Mormon literature as an alarm, the human announcement of war or the announcement of a message from the skies.  Masonic symbolism in the vane is not discussed here.

Hosea 5:7-9  7 They have dealt faithlessly with the LORD; for they have borne illegitimate children. Now the new moon shall devour them along with their fields.  8 Blow the horn in Gibeah, the trumpet in Ramah. Sound the alarm at Beth-aven; look behind you, Benjamin!  9 Ephraim shall become a desolation in the day of punishment; among the tribes of Israel I declare what is sure.

Indicating the cultural context for the sounding of the horn/trumpet:  Daniel 3:15  15 Now if you are ready when you hear the sound of the horn, pipe, lyre, trigon, harp, drum, and entire musical ensemble to fall down and worship the statue that I have made, well and good. But if you do not worship, you shall immediately be thrown into a furnace of blazing fire, and who is the god that will deliver you out of my hands?"  This shows the breadth of the trumpet announcing a judgment of sorts in the gentile world prior to its image being used for apocalyptic eschatological purposes.

Recall that the horn was to be used to signal battle formations and strategies in the War Scroll of the community at Qumran.

The book carried by the angel could reference a number of possibilities, including the Book of Life mentioned in Revelation - or alternatively - the bringing forth of the Book of Mormon:

1. The Book of Life: e.g.
1. 15 And whosoever was not found written in the book of life was cast into the lake of fire. 1 And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.  2 And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband. Rev 20.15-21.2 KJV.
1. The New Jerusalem was understood by the Mormons as a reference to a literal gathering of Israel - of whom they saw themselves as descendents.  This gathering was to take place prior to, and facilitate, the end of the dispensation.
2. Nauvoo was the second great gathering place of the Latter-day Saints.
2. or The Book of Mormon
1. The BoM begins with a vision in which Lehi is carried away and receives a book from heaven which he reads on the spot which prophesizes of the destruction of Jerusalem. 1Ne 1.7-14
2. "Behold . . . for the hour cometh that I will drink of the fruit of the vine with you on the earth, and with Moroni, whom I have sent unto you to reveal the Book of Mormon, containing the fulness of my everlasting gospel; to whom I have committed the keys of the record." D&C 27.5
3. "at that day when the book shall be delivered unto the man of whom I have spoken," 2 Ne. 27.12
4. "a Bible, a Bible, We have got a Bible, and there cannot be any more Bible" 2 Ne. 29.3.
5. "Wo be unto him that shall say: We have received the word of God, and we need no more of the word of God, for we have enough!"
6. "And now, I would prophesy somewhat more concerning the Jews and the Gentiles.  For after the book of which I have spoken shall come forth, and be written unto the Gentiles . . . there shall be many which shall believe the words which are written; and they shall carry them forth unto the remnant of our seed. And then shall the remnant of our seed know concerning us, how that we came out from Jerusalem, and that they are descendants of the Jews.  And the gospel of Jesus Christ shall be declared among them; wherefore, they shall be restored unto the knowledge of their fathers, and also to the knowledge of Jesus Christ, which was had among their fathers" 2Ne. 30. 3-5 (classic ex-eventu prophecy of the story of the Book of Mormon).

The Book in the hand of the angel could also be a reference to the revelations and prophesies revealed to Mormon Prophet Joseph Smith as recorded in The Book of Commandments (BoC) which was later revised into the Doctrine and Covenants(D&C). 

1. Perhaps the trumpet in the angel's hand symbolizes that the gospel shall be declared "as with the voice of a trumpet, both day and night" (D&C 24:12), and that the Lord has commanded his servants "to declare [his] gospel with the sound of a trump...unto a crooked and perverse generation" (D&C 33:2).
2. concerning the signs of my coming in the day when I shall come in my glory, in the clouds of heaven, to fulfill the promises that I have made unto your fathers BoC 48.15-16.
3. "And now, verily saith the Lord, that these things might be known among you, O inhabitants of the earth, I have sent forth mine angel flying through the midst of heaven, having the everlasting gospel . . . and this gospel shall be preached unto every nation, and kindred, and tongue and people.  And the servants of God shall go forth, saying with a loud voice: Fear God and give glory to him, for the hour of his judgment is come"  D&C 133.36-7.

Other relevant citations from the Bible:

1. "And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other."  Matthew 24:31.
2. Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,  52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed. 1 Corinthians 15:51-52.
3. And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, Rev 14:6.
1. this "other angel" is interpreted by Mormons to be the angel Moroni - the book is the Book of Mormon, or as Rev 14.6 calls it, "the everlasting gospel"