Put simply, the best books are those which have been polished to perfection by their writers: think Flaubert, Zola, Balzac and many other French writers. Don't think Jeffrey Archer, Grishman, Barbara Cartland or any of the other atrocities of commercial writing. Had the latter writers spent more time on the 'polish' they may have avoided some of the hackneyed descriptions and continuity errors that litter their pages.
Then again, maybe they wouldn't have sold as many books, either. Perhaps I am proposing a new category entirely: 'Hick Lit'!! Churn it out, smother the world with it featuring a garish, sexy cover, stick to a theme and there you have it! A runaway success. Or a 'runaway jury'. I think that many of the latest vampire books fall into this category as do many YA tales which are presently flooding the markets. These books are filling shelves that could hold better written stories!
It is not, strictly speaking, true to say that the market decides what is good or bad. After all, McDonald's, KFC and their ilk are amazing franchises purveying total crud and look how quickly they tapped into the collective base instinct of man to be lazy and greedy! The Harry Potter franchise is pretty much a parallel to this: a supersized kids' book with less real content than it should have. But successful? Heck yeah!