This book will discuss the final judgement as explained and revealed in Scripture. Scripture of course that all rational men will admit is divinely inspired, unless they are blinded by sin and stubbornness.
God's judgment is a complex idea in Scripture, beginning in the Garden of Eden and stretching down through the present day and forward to the Final Judgment at the end of time. God judges both groups of people and individuals, as well as both angels and human beings. What's more, when God says He will judge on a given day, that can be Scripturally tricky, since "day" does not always mean "24 hours" in the Bible, but can sometimes mean "stretch of time" or "period of time." What Augustine is going to write about in this book, however, is only the last judgement:
When Christ will come from heaven to judge the living and the dead. This will be a day of judgment in the precise sense that there will be no place for any uncomprehending complaint that this sinner has been blessed or that that good man has been punished. On that day, we shall see plainly the true fullness of felicity of all the saints and only of the saints, as we shall see the supreme and deserved misery of the wicked and the wicked alone.Chapter 2:
When we look at this world, we see an incomprehensible jumble of good and bad people and rewards and punishment. That is, the guilty are set free, the innocent punished, the wicked get rich and the virtuous fall into poverty--sometimes. Sometimes we do see things work out the way justice suggests that they should, but many times we do not. What we must learn is that we cannot know what God is doing simply by looking at the world as it is (and so "it is... good for our souls to learn to attach no importance to the good or ill fortune which we see visited without distinction upon the good and the bad").
All of this changes at the last judgment. There, the wicked and the righteous will be separated and the true nature of God's character will shine forth as He straightens out the world and sets things to rights.