What series like Twilight do for the field of literature is systematically devalue the integrity of the educational value that literature provides. Quality writing is anything that provokes the reader to think and to furthers the cause for provocative thought. Writing should provide the reader with anything that can lead them in the direction of higher thought. Plus, the Twilight series does not even contain anything aesthetically pleasing but just a palate of generic hopeless romanticism that ultimately inspires the youth of my generation to misconstrue what the parameters of a healthy romantic relationship are.
Before the 19th century, books were costly and the main audience were really the upper classes (since they could afford it). Since the wealthy were educated, the books they read contained complex literature styles that would be difficult for today's common mainstream reader. Books like Frankenstein, Wuthering Heights, and The Sound and The Fury contained different literary styles and have complex vocabulary.
However, as time passes, books become available to a wider range of audiences. Thus, the quality expanded from complex to simple-to-complex. Words have to be simpler so the everyday person could comprehend most of the books' content. This is not bad, however, because books with simplicity often contain everyday morals, such as in Dr. Seuss's books.
Actually, I have read the books before the movies came out and the writing is actually good. It's is very well written. The movies really brought down the books. The books are good, the movies? Not so much.. I still have hope for future writers. Movies always make the books stereotypical.
The issue is not that we are lacking in writing value. The problem is that in the information age more people use computers and less read, thus we have e-books and Kindles. This writing is not devaluing our writing quality. The quality of writing as a whole has declined from proper English to slang and shorter words because people's time is important to them and the quicker they can get a point across the quicker they can get to other things; thus the use of texting slang and abbreviated speech. The quality has declined to meet the standard as a whole to keep people interested. An interesting story with easily understood words is more likely to sell than a treatise on economics.
The quality of a book on a series, or how well it sells, does not say a lot about all readers in society. Twilight is a series that many people enjoyed, but many of the fans of that series still enjoy more challenging books. You cannot just make assumptions that everything is dumbed down.