A new book, and it introduces a new body of law, mainly to do with sacrifices. In those days the sacrificial system had two roles. On the one hand, it was a system for worshiping and interacting with God. People like to give, to do something, either to celebrate or to ask for forgiveness, to thank or to atone. The sacrifices enabled people, rich or poor, to feel part of the religious system, to contribute to it, and to feel closer to God. Sure there was prayer and meditation, but people seem to need to feel part of a wider community as well.
The Temple ceremonial consisted of community ceremony, daily, weekly, and throughout the year, as well as on festivals. But it also allowed individuals to come in at different times and in different ways to be part of the ceremonial. In a way it is a bit like synagogues now. You can come in to be part of the crowd, or you can come in to do something, to participate as an individual, as well.
Not all the sacrifices were animal. But they were indeed a major part of it all, for the simple reason that originally animals could only be killed for food in a special way, in a special place, and by specified people. This way killing animals became a religious ceremony rather than a dehumanizing outlet for aggression or cruelty.
So originally if you wanted to eat meat, you had to go up to the Temple, have the parts you couldn’t eat offered up and burnt together with incense, and the rest divided between you and the priest (taxation!). The whole procedure was far more dignified and restrained than the processing lines and force required in modern abattoirs, which we hide from normal view. Nevertheless, the Gemara records animals fleeing the knife only to be turned back. Perhaps once the Temple is rebuilt, Elijah will replace the animal sacrifices with the meal and vegetarian options that actually existed then too.