I've always hated that "don't judge a book by its cover" phrase -- if a book says PROBLEMS IN QUADRATIC THEORY on the cover, it's highly unlikely to be a biography of Abraham Lincoln.
Sorry, but in relation to books I have seen this happen time and time again.
The problem arises once the PR and marketing departments get involved. I was in publishing for a long time and the number of times I saw cover artwork changed not to reflect the subject matter, but rather what would move units (and was therefore unrelated to the subject matter) occurred often.
"You can't judge a book by its cover" holds a great deal of truth funnily enough in relation to books.