Book cover design.

Don’t judge a book by it’s cover, design it instead.

Needing no extra explanation the title is the most valuable source of information for the reader. After all, you need to know what the book you’re about to read is called.
Usually you’ll see some accompanying text with the book cover. This can be referential or a clue as to what the book details to help contextualise the plot a bit more for the reader.
As with the title, it is essential to have the name of the author on the cover of your book. I’ve seen this designed in a number of ways. From being smaller than your title to being larger the layout varies but you cannot have a cover with no author.
This could include the main subject of your cover or a character involved. Something else for the reader to focus in on.
The colour you use and how you apply the finishing touches to your design. For the most part ensuring a strong level of contrast within your design will help it to stand out amongst a crowded book shelf.