The book tells the story of two boys, one of them 12 years old, the other 36. 12-year-old boy is Marcus. He lives with his single suicidal mother. He's a bit... eccentric, and struggles in his school and family life. 36-year-old boy is Will Freeman. He's "cool". He's rich without having to do anything. He likes women, luxury, having a lot of CDs. He doesn't like taking responsibilities. Lives of these two boys cross in a strange way, when Will's single mother girlfriend has to baby-sit Marcus. The book is set in London in 90s, and the title is a reference to the famous Nirvana song "About A Girl".
Through the book, Marcus learns how to be a child, and Will learns how to be an adult. And what makes their story special is that they learn these from each other. They make such a cute couple.
Nick Hornby succeeded at narrating the story from two different point of views. While you read the book, you really understand the minds of Marcus and Will. Also, they're really interesting and lovable characters, despite their seemingly not-very-appealing attitudes. You know they're not evil, they just behave like that because they don't know any better. But they start to learn, and that's why you're loving them.
The book doesn't get boring at any point. The turn of events are carefully planned and almost always surprising and enjoyable. It's a really good read if you want a modern fiction book.