What makes a bad book?

AmericanBookReview.org has published a series of 40 small essays on the subject of bad books, which names some names, some of which don't strike me as all that bad, although others might disagree. Some, on the other hand, are dreadful. One does wonder why some books remain popular despite their seemingly objective lack of quality, but how objective is quality, anyhow? Here's the pdf of the list.