The first one I thought of was Saffy's Angel (which won the Whitbread Children's Award). I enthused about it to the lady and she had a read of the back and decided to take it (and the first sequel, Indigo's Star). But she did say that she wouldn't have picked it up because the cover was so horrible. "It looks like one of those cheap trashy chick-lit girlie books, you know" - I didn't know. I LOVE all the Casson Family covers. All I could say was "I suppose it is a bit..." and then trail off. I wanted to finish that sentence with "...beautiful" but couldn't.
Then today I was putting some books out and spotted a copy of The Darling Buds of May with a completely hideous cover. If my parents had had these editions there's no way on earth I'd have read them! They have the editions with cartoony covers, like their copy of Animal Farm had - which is why I picked it off the shelf at age 9. 1984 didn't have such a nice cover, but I read that next because I'd enjoyed Animal Farm so much.
Covers can indeed make or break a book. There are certain types of cover that I wouldn't pick up in a million years, and I may possibly be missing some good books because of it. We actually have a display at work that I set up called "Books that make you go mmm..." It's not a display of porn, it's books with beautiful covers.
These are some beautiful books.