The impact of books depends on the reader.
Books can change lives. Different books have different impacts. There is not one simple answer. All people are different and so is their reading preference.
Sometimes books with similarities to an individual's life make the most impact. Seeing how a similar life was lived can inspire a person to achieve their dreams, be bold or not feel quite so alone. Settings also can have an impact. A vivid look at the world outside your view can inspire just like a familiar character.
In short, the impact of a book is dependent on that person. The most impactful books are the ones no one expected to change their lives.