A good summary and interpretation of the sources (and later folklore) surrounding Rosh Hashanah comes from from Howard Schwartz’s Tree of Souls:
All things are judged on Rosh ha-Shanah, and their fate is sealed on Yom Kippur. Some say there is a ledger in heaven that records all that has taken place from the time of Adam throughout the generations. The ledger is open, and the hand is writing every single thing that a person does below. Whose hand is this? Some say it is that of an angel. Others say that the hand belongs to God Himself, and that the book is the book that God is writing. That is the meaning of the verse And a scroll of remembrance has been written at His behest (Mal. 3:16). This is the Book of Life.
In addition to the Book of Life, there is a second book, the Book of Death. During the Days of Awe God’s scrutiny of our lives is intense, and it is to be hoped that if our name has strayed to the wrong ledger, God will say, “I have removed your name from the Book of Death and put it in the Book of Life, as it is said, For Yahweh has redeemed Jacob” (lsa. 44:23).
Others say that there are three books opened in heaven on Rosh ha Shanah, the New Year: one for the wholly righteous, one for the wholly wicked, and one for those who are neither completely righteous nor completely wicked. The wholly righteous are inscribed at once and sealed in the Book of Life; the wholly wicked are inscribed at once and sealed in the Book of Death; and the fate of the intermediate is suspended from Rosh ha- shanah until Yom Kippur, the Day of Atonement. If they repent and are found worthy, they are inscribed for life; if they fail to repent, they are inscribed for death. Yet so great is the Power of atonement on Yom Kippur that it is said to bring about atonement even for those who have not repented.
When God sits on the Throne of Judgment, the Books of Life and Death are open before Him, as it is said, The court sat and the books were opened (Dan 7:10). His garment is as white as snow, the the hair on His head pure wool, and His cloak seventy times brighter than the sun. A pair of angels, both named Shofariel, are the keepers of the Books, which are closed to everyone else. No other angels have access to the secrets inscribed there. Not even Metatron, the Prince of the Presence, is permitted to peer at that divine list. Among the sages, only the Ari knew how to peer into those secret books. In this way he learned the fate of his followers from the first of Rosh ha-shanah.
Some say that the world is not only judged on those holy days, but that God sits upon the Throne of Judgment and judges the world every day. The Books of the Living and the Books of the Dead are opened before him, and all the children of heaven stand before him in fear, dread, awe, and trembling. And in this world as well every being trembles before the eyes of God.
Others say that God is a merciful God. Even as clouds are swept away by wind, so the iniquities of Israel are swept away in this world, as it is said, I wipe away your sins like a cloud (lsa. 44:22). For from the time He created Adam, God has known that if He held mankind to account for its successive misdeeds, the world would not endure. Therefore God remembers those who observe the Torah, but puts those who commit misdeeds out of mind. This means that God removes their names from the Book of Death and puts them in the Book of Life.
Commentary by Howard Schwartz: There is a series of myths about heavenly books. Some of these are described as ledgers in which God keeps track of good and bad behavior. The best known of these myths concern the Books of Life and Death. The talmudic version lists three books all linked to Rosh ha Shanah: the Book of Life, the Book of Death, and a book concerning the fate of the intermediate. However, for most Jews there is a conscious focusing on the Book of Life, while the Book of Death remains for the most part unnamed and largely unmentioned, and the notion of a third book has essentially vanished from the tradition. The current understanding is that a person’s name is inscribed on Rosh ha-Shanah either in the Book of Life or the “Other Book,” (i.e., The Book of Death), but the name is not sealed until Yom Kippur. This is stated succinctly in the Talmud: “Man is judged on Rosh ha-Shanah and his fate is sealed on Yom Kippur” (B. RH 16a). Altogether, these ten days are known as the ten Days of Awe, and they serve as an intensive period of self-examination and repentance, climaxing on the day of Yom Kippur. There is also an element of negotiating with God, as Abraham did concerning the fate of Sodom. The hope, of course, is to obtain God’s forgiveness in order to change a negative fate. According to the Talmud (B. RH 17b), “Great is the power of repentance; it can rescind a person’s final sentence.”
There is a biblical precedent for these heavenly books, found in Jeremiah 17:1: The guilt of Judah is inscribed with a stylus of iron, engraved with an adamant point. Another reference is found in Malachi 3:16, And a scroll of remembrance has been written at His behest. In Esther Rabbah 2:23, this is referred to as the Book of God.
It is important to note that these heavenly books are not to be confused with the Torah. The general view is that they are for God’s eyes alone. These other books were never handed down from heaven, as was the Torah. The one possible exception is The Book of Raziel, said to have been given to Adam by the angel Raziel. However, The Book of Raziel can be seen as a substitute for the Torah, until it was given at Mount Sinai.
Another heavenly book is described by Ezekiel, who has a vision of the semblance of the Presence of the Lord (Ezek. 1:28), and hears a mysterious figure speaking, who tells him to “open your mouth and eat what I am going you” (Ezek.2:8): As I looked, there was a hand stretched out to me, holding a written scroll. He unrolled it before me, and it was inscribed on both the front and the back. (Ezek. 2:9-10). Since scrolls are traditionally written on only one side, Ezekiel seems to be describing a new kind of book, one relevant to the inner, spiritual life, as well as to day to day existence in the world.
Zechariah has a vision of a flying scroll, presumably of heavenly origin: “What do you see?” he asked. And I replied, ‘A flying scroll, twenty cubits long and ten cubits wide” (Zech. 5:1).
The description of God seated on His Throne of Judgment derives from Daniel 7:9. 1 Enoch 47:3 has a vivid description of God seated on the Throne of Glory, with the Books of Life and Death open before him, and all of God’s counselors standing there.
3 Enoch describes a pair of angels, whose full names are Shofariel YHVH Memit and Shofariel YHVH Mehayeh. They are the keepers of the Books, which are closed to everyone else, even Metatron, who has access to virtually everything else in heaven. (Note how the names of these angels contain the Name of God, hinting that they are an extension of God.) More common, however, is the tradition that Metatron is the heavenly scribe, and he sits on a throne in heaven and writes down the deeds of Israel. Elisha ben Abuyah (Aher) is said to have experienced great shock when he saw Metatron thus seated, which led to his exclamation, “There are—God forbid—two powers in heavenl” (B. Hag. 15a).
In Babylonian literature, the gods were said to possess “tablets of destiny” containing the fate of all mortals. These tablets were adjusted on the New Year, when Marduk, the chief among the Babylonian gods, was said to cast lots in heaven to determine the fate of humans. Thus it would appear that the Jewish myth of the Books of Life and Death finds its source in this Babylonian tradition.
All in all, the portrait of God found in 3 Enoch is very anthropomorphic, on one hand that of an old man, with white hair, described as wool, and at the same time the hint of a ruler dressed in armor, prepared to do battle. 3 Enoch 3:1-10 also describes four angelic princes known as ‘Irin and Kaddishin who stand around God like court officers. These angels argue every case that comes before God.
While sitting on His Throne of Judgment, God tends to deliver harsh judgments, as at the time of the Flood or with Sodom and Gomorrah. That is why everyone in heaven is trembling as well. God’s judgments of angels who show the slightest hesitation to obey his command is swift—they are cast into a flaming river and cease to exist. Likewise, seven planets that did not manifest themselves when commanded to do so are punished in the fires of Gehenna.
From the first of Rosh ha-Shanah the Ari is said to have known the destiny of his followers, though he usually kept it secret.
Rabbi Shimon ben Lakish raises the question of whether there is actually such a heavenly book, saying, “Does God really have a book, and in the book he writes? That is to say that everything is known and revealed to Him, as it is said, For His eyes are upon a man’s way; He observes his every step (Job 34:21).”
This image of a time of judgment has proved to be haunting for all Jewish generations. Even secular Jews find their way into the synagogue on Rosh ha-shanah and Yom Kippur, when God’s intense scrutiny can be felt by all.
– Howard Schwartz, Tree of Souls: The Mythology of Judaism, 289-291