I always judge books by their covers - that's what covers are for! You look at the image they chose to use, read the blurb on the back, and decide whether you want to read it.
(Yes, I know - the phrase means "Look past surface details and see inner beauty" and so on. It's just that for books the heuristic works pretty well).
It doesn't really, because a book cover is advertising, not information - it doesn't really tell you any more about the contents of the book than the pictures of food on a fast food menu accurately represent what you get on your plate.
A book with a cover is a collaboration of artists. Perhaps forced by the publisher, that the author and painter would never attempt on their own, but a collaboration nonetheless. Together, they have created something more than the sum of its parts.
And while it's true this can neither vouch for nor indict the writing itself, the rest of us are allowed to enjoy the art just the same.
Besides, the new digital era debunks your claim... cover images are still included in epubs even if those do not sit on the wire rack next to the cashier at the store.
Marketing material is often the best reference you have to discover what something is about. Also, the pictures of food on a menu usually inform you better what you'll get than a realistic picture.
Marketing has many functions, a large share of them are hindered by misleading you. A few are not. The result is usually very transparent (but effective).
That's actually a good analogy, because the pictures of food on fast food menus are still useful in most cases. You can see the lettuce poking out and ask for it to be removed, or ask for extra cheese.
The way a book is advertised absolutely tells you about the book, in most cases. There is a series of thought and decisions which go into making a cover, including finding a compatible artist and giving them a solid brief.
As with fast food, there are exceptions... But they are exceptions. It may be more of an heuristic than a rule, but it's undoubtedly often useful. The cover is part of the book, and an important part; both up to and beyond the advertisement.
It’s not a good analogy. Fast food images show the sandwich, even if a better version of it. This isn’t always the case for books, for instance in the expanse series the authors stated they write the book and send it off and then the publisher picks an image which has little relation to the books plot and was solely chosen to look cool alongside the books name.
(Yes, I know - the phrase means "Look past surface details and see inner beauty" and so on. It's just that for books the heuristic works pretty well).