When it comes to designing a book cover this is also true. The cover won't tell you if the book is well written, if the story is carefully constructed and edited, how much research has gone into writing it but it will tell you whether it is the book for you. A good book cover should attract the reader's attention and convey certain messages: what sort of story it is, what the reader can expect: romance, mystery, crime, murder, erotica, historical. It should never mislead the reader or promise something that it is not.
Although it is perfectly possible to design your own book cover, it is often easier and more successful to buy the art work from a dedicated site such as SelfPubBookCovers.com. That is what I did for my latest novel, a mystery story set in the Spanish resort of Marbella.
The book is now available as an ebook on Kindle, Apple iBooks, Barnes and Noble and in paperback from most on-line bookstores, including Amazon.