Book 13 deals with God the Spirit as he juxtaposes the story of creation with the history of the Church. We'll come back to this structure frequently to fill in some of the gaps and flesh it out. But for now that will be enough to give us some idea of where we're going. Book Two is the book of love gone wrong. And that is what sin is for Augustine.
Sin is love that is misdirected.
His analysis of sin in Book Two is quite interesting, and I think it is usually misunderstood. In particular, I have never read an interpretation of the pear-tree episode that seemed right to me. So what I want to do first is to set up the general analysis of sin that Augustine offers in this book, and then see how it applies to the theft of the pears. Think of it like hole in a shirt. A hole is not an actual thing.