A confession is a book in which someone tells a really long story about something they did that they're not proud of. It's kind of like when you tell your mom or dad that you broke something or did something bad, but way more serious.
The person who wrote the book tells the story to try and feel better about what they did and to maybe make up for it in some way. Sometimes they might also want to warn other people not to do the same thing they did.
The story is usually really detailed and goes on for a long time because the person wants to explain everything that led up to them doing the bad thing. They might talk about their feelings, what other people did that influenced them, or even stuff that happened a long time ago.
Even though it's not a happy book, some people like reading confessions because it helps them understand why people make certain choices and how their actions can affect others. It's important to remember that just because someone did something bad in the past doesn't mean they're a bad person forever.