Do you read the inside flaps that describe a book before or while reading it?
Sometimes before. Sometimes during. Sometimes after.
I'm always tempted to read them, and sometimes I do. But they often contain spoilers, which I hate. And they shape the way I experience the book, which I don't like. Additionally, sometimes the supposed blurb actually contains an interpretation of the story that I disagree with, and that's aggravating to me.
So, the best time for me to read the flap is after I've read the book. Then I've experienced the book on my own terms, and I'm ready to argue with the flap if need be. :-)