The reason for this point is, books--written words in general--are inherently human things. (Some of you may be calling out "Objection! MegaHAL! Eliza!" Please kindly hold your questions to the end.)
Books are written by humans, and they are read by humans. And no human exists in a vacuum--everyone is a product of his or her society, and its attendant baggage.
This means that writers bring their own unspoken assumptions about people around them--not only about groups like race, class, gender, and so on, but more subtle and difficult-to-define things like how given people would react in a given situation--to the books they write.
It flavors the book, much in the same way that liquor is flavored by the wood of the barrel in which it ages.
And this means that readers bring their own unspoken assumptions about people around them to the book when they sit down to read. Times change--the reader who sits down this afternoon to read, say, Henry V or Jane Eyre or Gulliver's Travels will not necessarily have the same web of cultural assumptions and supports as the readers who sat down to these books a century or two ago.
There's also an additional dimension, though, and that is the expectations and/or moods of the reader. And these change from decade to decade and individual to individual.
Because of the inescapable human dimension to books, we cannot avoid talking about the ways in which humans relate to each other when we talk about books. And another way to say "the ways in which humans relate to each other" is to say "politics." This is neither a good thing nor a bad thing, but it is a fact, and must be allowed for in the way that someone tossing a frisbee must allow for the wind.
And that's what Postmodernism is all about, Charlie Brown.