I love books covers and everything about them... most of the time. So I found an article talking about the whole design of the book cover and who decides what the covers should be and it turns out that the authors don't have any say in the cover for their book and it's the editors and designers choice (who are "trained" to know what catches a persons eye). Which surprises me since some book covers are bad and you cant even read the title of the book sometimes.
Don't judge me but I do sometimes judge books by their cover just because I think they're pretty and would look good on my bookshelf. But sometimes, it looks like someone had a little to much fun with photoshop. For me, simplicity catches my eye because it gives an air of mystery.
A great color scheme works to. The Beautiful Creatures covers have a simple color scheme: black and purple, black and blue, black and gold, black and green. It doesn't change with a different book. Theres a balance and it flows well together.
Matt Taylors designs for books are amazing also. Simple yet effective. They are colorful, not overbearing and you can read the title!
I'll admit, there are some book covers that go well with a lot of things going on in it like the cover for Across The Universe by Beth Revis because space mystery! It doesnt feel over bearing yet theres a lot going on. 
I also feel like if you want to go all out then draw it. Like Harry Potter covers, they're magical and drawn very well. Ruben Toldeo, fashion illustrator, did some book covers for Penguin Classics books which I think are very beautiful and yeah I had to buy the Jane Eyre one so sue me. He also did Wuthering Heights which I would actually read just because book cover.
Overall I think the design of a book can make or break sales of any book. The whole deign from the colors to the type that they use. It's like a first impression.
