Being a writer, I started off a million miles away from anything design-related. I think in words, not images, and of course there's the proverbial "don't judge a book by its cover". Nevertheless, I had to come to terms with facts: readers who come across your book for the first time see the cover. You have about a microsecond of their attention before they decide, "This isn't for me" or "I want to find out more". Clothes are just a cover, too, but nobody would think of showing up on a gala night in a bathing suit (I hope!).
Having finally understood the importance of cover design, I could congratulate myself on having gained another point over which to fret, all the while wishing I had hundreds of dollars to spend on a brilliant custom-made cover by a top professional designer. Of course, money being a big stumbling block for me, I have to use hacks such as premades on sale, cheap or free stock photography, and Canva.
Lately, reading Derek Murphy's ebook on cover design has revolutionized the way I think about book covers. It's absolutely packed with sound, down-to-earth, straightforward, practical advice. And the best part? It's free! You just click the link above and grab it. It doesn't get better than this.
In a nutshell, here is the main idea: your book cover doesn't need to be brilliant. It only needs to be good enough to make people read the description. A cover doesn't need to be so expensive it breaks the bank, or so original that it's hard to guess the genre by looking at it - the genre has to stare into the reader's face in that millisecond of their attention you have - or so complex that people are forced to stare for long minutes trying to figure out what goes on there.
What I especially loved is the message that anyone can come up with a decent book cover, even authors on a shoestring budget that can barely afford Shutterstock. In contrast to other guides, this book doesn't scoff at authors who DIY, patronizingly saying, "come back to publishing when you have more money". You can learn enough to come up with clean, simple covers that do the job well enough until you can upgrade.
Derek walks us through it all - layouts, fonts, colors, genre tropes, with multiple case studies and examples of what not to do. So if you have time to just read one book on cover design... pick up this one. He also has plenty of other stuff about writing and getting your work out there.