As many may know, there are two types of book cover designs, each have different feels and level of engagement. The first is

The Regular Book Cover Design :
This book cover makes use of pre-existing elements creatively blended together by a good graphic designer. The elements include photos, text and shapes. What’s good about this kind of book cover is that it is usually more realistic and professional looking when done right.
Choosing what kind of cover your book requires is mainly based on the audience you wrote it for. The cover of the book sets the tone before the content. Are you writing an eBook, a factual or practical book, a book for more matured audience? Then you are more likely to require a Regular book cover design.

Example below (I would be using a cover that I designed myself)

www.nairaland.com/attachments/11017011_warsawfrontandback1_jpeg5f572d7bdfe88af81c5765e6219d1828

Illustrated Book Cover Design :
The second type of book cover is the Illustrated book cover design, it is design by an artist or digital illustrator actual drawing the elements in the cover. Things like sky, people, objects etc. are drawn and painted digitally. The greatest advantage of the illustrated book cover design is that it is easily molded to your requirements. All the designer has to do is draw what you want, unlike the case of the regular book cover design where more often than acceptable, a pre-made image dictates how a cover would turn out. The Issue of copyright infringement is also removed since the book cover is drawn from scratch. The disadvantage is that it is usually less realistic looking since it is not based on actual photographs taken.

Use the Illustrated book cover when
*Your book is for children or young adults (the level of detail may differ for different age ranges)
* You are telling a story (Novel / works of fantasy or science fiction)
* You are placing entire new world /object or a very physically unique character that you want people to be intrigued by on the cover

Example below (I would be using covers that I designed myself)

[img]https://1.bp..com/-16xrZUVuVLM/XkXfZ06J8DI/AAAAAAAAAvE/nOwXn17SnCAmW0ci2xl2qoVg8la5tHS-wCLcBGAsYHQ/s640/Bible%2Bawokenedd.png[/img]

[img]https://1.bp..com/-eziBOA6VT9g/XkXfoMR0QsI/AAAAAAAAAvI/KrALaZtKEJYa-1ZG9MiY9db0GRq0Q2BeACLcBGAsYHQ/s640/Bible%2Bawakened.png[/img]

www.nairaland.com/attachments/11017008_0925_5x8_5standingpaperbackbookmockupcovervaultcopycopycopy_jpeg8804645c8aa0971df04d23f6d043ba14

[img]https://1.bp..com/-ykaIgfW7Qkk/XkXezsOZweI/AAAAAAAAAu0/EnfqGBvHfAQRYZIzC0QQArV9HdkY6YEkwCLcBGAsYHQ/s640/sketch%2Bbook.png[/img]

[img]https://1.bp..com/-5_vXdfipGCA/XkXehijFG9I/AAAAAAAAAuo/DpNSuk0TdBMUFW-LQAVND1E2yuwPq3wHACLcBGAsYHQ/s640/Wasika%2BCover.jpg[/img]

[img]https://1.bp..com/-h2hGaJ9N2rE/XkXecnDRGEI/AAAAAAAAAuk/lpIputLUuWosuP53Vf8WpYIVj4t8uvMJgCLcBGAsYHQ/s640/WASIKA%2Bmockup%2B%2528Copy%2529.jpg[/img]

It is important to note that these are not absolute rules, they are more like guidelines and can be broken and bent whenever necessary. There are also cover that fall in-between the two types, making use of both pre-made elements and actual illustrations at the same time.