Could it be that instead of a book "putting an idea in our head" when we read, that actually more technically, it is "getting an idea out of our head?" This book explains that the experience a reader has with a character is far less concrete than one might believe. Sometimes, it feels that ideas are being put there and our mind just accepts them, but Mendelsund shows that actually, the reader is using their imagination to follow along, and that participation is what he describes as the "reader's experience" so to speak.
Knowing that, a person can become more skillful in their art, because sometimes, writers make use of beautiful detail, but the detail is disconnected from the heart of the experience—the character's long decision-making process. In other words, Mendelsund feels that writers should help their readers to construct the right psychic ideas by evoking ideas through association.
That's the crux of his literary opinion in this book: Mendelsund shows that characters are tools used to evoke thought patterns in a reader's head. As the character sorts through a hypothetical fate, the reader learns tools and approaches that they might not have encountered before. Therefore, since that is the technical explanation of what actually happens, artists can focus on improving their magic-like ability to evoke ideas in their reader's minds.
Another way to simplify all of this is just to say that as the writer presents ideas one word at a time, or one phrase at a time, the reader imagines alongside them, hypothesizing about what the character might be experiencing. This is technical, but ultimately, it is supremely helpful if the reader ever wants to write for others—by understanding the art of literature in its technical detail, hopefully, writers will be more strategic and effective.
