[Anna Drake]

I've read it's good to study the covers of popular books in your genre to learn what sells your kind of book. The other piece of advice I recall is to include things on your covers that people expect from traditionally published books. That would include author tags and tags following the book's title. Readers expect to see those things. They've been conditioned to it from the way things are done in publishing houses -- or so goes the advice I've encountered.

If your book is like the Plum books, then your covers should look something like hers as well. And I think you've zeroed in on that idea. But experts say the most important thing is that the book's title and author name should be large enough to be read in the small postage stamp size most books are displayed in online. And that your typography should be creative and interesting. I suspect the numbers are large on her covers because they are the book's title.