Writing a book is a wonderful achievement. Getting it discovered is wonderful, too. It's probably a bigger challenge. The problem is that the finishing line for these efforts is very different.
In the first, the writing, you complete the book. No more work is possible. The success lies in the eye of the buyer. Very subjective.
In the second, the success lies in the sales numbers the author plugs into a spreadsheet as well the deposits and expenses related to generating discovery. The work goes on forever, and it is never complete.
"I wrote this great book. Some people agree."
"I'm selling my book. Some people are discovering it."
The difference is "I wrote" versus "I'm selling." The joy of having created, versus ongoing work.
Creating and its joy might not be the only reason you're writing a book. You might want others to discover it, buy it, recommend it, connect with you. You need both parts to be published. If you like commerce and relationships, you're suited for the second. If you don't find those fulfilling, you're suited for the first.
But if you have something to say that only you can say -- a unique voice, even more than a unique story — you'll be motivated to learn the second part to the make the first part more rewarding.
One wise answer to “What can one-book authors do?” comes from The Book Designer website. Buried in the comments are these three commandments
The article goes on to say
You come up with great titles by studying your genre and/or similar books to your own.
There are plenty of affordable book cover designers. There’s no excuse for not paying for a good cover when you’re not a professional designer yourself.
There are plenty of writer forums on which you can ask for help with a blurb. If you can’t afford to pay someone to write one, ask for help.
Finally, spend the time getting to know your audience, the influencers in that area of interest, and the places online where they discuss their common interest.
It’s fine to not want to learn marketing. Instead, take the time to build relationships with people whom you want to read your book. Also, make sure you’re creating a book they would want to tell people about.