There's no doubt about it; a flashy outside can really change how even an ordinary product is sold. Advertisers have known this for decades. And book covers and/or dust jackets have been playing this ad game for the last century or so, too.
But sometimes we get fooled. Sometimes the cover is far, far better than the book.
I was totally blown away by this cover, for example:
Isn't that gorgeous? It's stunning.
But I wasn't at all drawn into the book. After a few chapters, I tossed it aside, grateful I'd only gotten it from the library and hadn't spent money on it.
Also, the book labels itself as YA, but it isn't; it merely has a youngish protagonist. The first few pages alone have so many sexual references in them that it ruins the book for school bookshelves. Really, it's more appropriate for college-age beach reading.
In contrast, there's this book:
But the book is totally and completely wonderful. It's not labeled YA, but it's completely appropriate for young teens -- if you can get them past the cover.
As an indie author, I had to go through designing a book cover that I hoped worked well with the book, knowing very well that a lame cover would turn kids off, no matter what the book was like inside -- because kids especially DO judge a book by its cover.
What about you? Have you ever mis-judged a book by its cover, either for good or for ill?