From Lexicanum

The Book of Judgement is the legal code of the Imperium, enforced by the Adeptus Arbites. It has long ago spread beyond the confines of a single literal volume. It is a vast body of laws, rulings and precedents compiled over the ten millennia of the Imperium's existence, and ranging from the words of the Emperor himself from the days he walked among men, to the most recent decrees of the High Lords of Terra.

Sources

• Codex Imperialis
• The Long Arm of the Emperor's Law