In some books long descriptions are not annoying, but they must be used really well to not seem superfluous and misplaced. When they're overused, the reader starts thinking about other things, doesn't follow what's written in the book anymore and becomes distracted. I don't like when a writer uses descriptions to make the reader think about the greatness of his ability. That is completely pointless and I suppose also negative for the book, unless the book is something autoreferential or er... avant-garde? I assume we're talking of books with a narration, a story. Essays etc. work differently. In that case could be the fact you have to reflect on things that are completely external is a necessity. Anyway, most avant-garde books end being pretentious and irritating, and avant-garde writers don't have a clue themselves about what they're writing