If you’ve ever heard of barbershop quartets, you might know the effect known as ringing chord. It is a so-called fifth voice that is created by the effect of the four voices singing in harmony. Though this is a fascinating concept by itself, in this case I use it only as a comparison.
Like four voices creating an additional one, two minds create another, third mind. This is also known by the word “mastermind”. By brainstorming and discussing ideas, two or more people gain more understanding and/or meaning than the sum of their parts. In this respect they ‘create’ a third mind that they all have access to.
Now, to get to my point, reading a book is kind of a long distance mastermind. How, you ask? Well, though the writer and reader can’t discuss the ideas (well, they could, but the fact is that they don’t), the former shoots ideas at the latter and the latter then attaches meaning to it. In other words, the two create a bigger meaning for ideas and concepts than each could do on their own. There’s a quote from Neil Gaiman that I have to find quickly. Here it is:
"If you are pointing out one of the things a story is about, then you are very probably right; if you are pointing out the only thing a story is about you are very probably wrong - even if you're the author." - Neil Gaiman
The very fact is that there are a great number of meaning anyone can attach to any story. Both the writer and the readers attach meaning according to their own experiences, thus each will be somewhat unique.
But every reader’s meaning would have been impossible without the writer’s contribution. So in actual fact, the writer and reader are working together (long distance) to create a meaning that none of them could have created without the other.