The website Authors Wish, which is supported by POD publishing channel Ingram Spark, has links to various tooks that help authors do everything from writing to promoting their books.
Click on the green dots to go to different stages in the process. There’s a host of links including to Scrivener, about which it says:
It also includes links to professional services if you’re in need of help getting that book ready for public consumption. Under Design, you’ll find tools to help you create a good cover.
–Mike Perry