Jubilees, sometimes called Little Genesis or Lesser Genesis and known at Qumran as the Book of the Divisions of the Times, runs to fifty chapters. It is not a commentary. It is the narrative of Genesis 1 to Exodus 14 told again from the beginning, with material added, removed and rearranged, and presented throughout as dictation from an Angel of the Presence to Moses during the forty days on Sinai. The claim of the frame is that this account, not simply the one in the Torah, is what Moses received.
The rewriting has consistent aims. It closes the awkward gaps in Genesis: Cain's wife is named, so are the wives of the patriarchs, and the sons of Noah divide the earth by lot in a passage that becomes a charter for Israel's title to Canaan. It removes what is embarrassing. Abraham is a monotheist from youth who burns an idol temple. Above all it dates everything, placing each event in a numbered jubilee, week and year, so that sacred history acquires a continuous chronology it does not have in Genesis.
It also introduces Mastema, prince of the hostile spirits, whose demons are reduced to a tenth after the Flood at Noah's plea, and who prompts the testing of Abraham, so that the command to sacrifice Isaac originates in a challenge rather than in God alone. The festivals are pushed back into the patriarchal age: Shavuot is kept by Noah, Sukkot founded by Abraham. And the law is stricter than the Torah's. Sabbath-breaking and intermarriage carry death, and the rape of Dinah becomes the ground of an absolute prohibition on giving daughters to gentiles.