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. It's a lack of shirt where shirt ought to be.
Evil is the same way.
It's a lack of goodness where goodness ought to be. Now for Augustine, all goodness either is God or comes from God. God is of course the pinnacle and paradigm of goodness. Whatever is good in creation is good because it reflects in some way the goodness of God. Sin comes in when you try to have the goodness of a creature apart from the goodness of God.