I did a series of mock book covers for my portfolio recently since I would love to do some book covers in the future and I realized I had no examples with my current work!
Students and anyone else learning illustration: If you want to do something, you have to have examples because how else will the client know what to expect? You also learn along the way good methods to use so you can get rid of the bad ones and expand the good ones. It's best to learn this stuff on your personal work so when professional work comes around for that topic, you are prepared!
Here's the series of book covers I created for this project.