Wordpress themes are evolving to reflect modern internet usage, which means website navigation that worked for desktops is being replaced with conventions more suitable for cell phones. The old functions are still available, but WP is becoming less "blog" and more "e-reader" with every update.
Most paid themes will feature a flexible layout that adjusts your content to fit a cell phone, but you could focus your search to mobile themes, and at the very least you should test the themes on a mobile device because the way the internet is headed the vast majority of new readers will discover you through social media or search engine, and these are already dominated by mobile devices.
Use Pages (not Posts) to publish sections and chapters that should be read in a specific order. Posts are for content that will be searched and re-sorted by the viewer using category, date, or tag words.
Pages can have sub-menu pages. A "book" might be a top-level page with "sections" and "chapters" as parented sub-menu pages. WP will create the sub-menu automatically, but you can also create a Menu that features or hides this structure.
Longer Pages can be presented as shorter book-like pages with a nextpage tag (WP calls it "pagination"), and your WordPress theme will handle the navigation.
Use Posts for related articles, worldbuilding content, in-store appearances, personal updates, etc. Anything that supports the book that isn't part of the text should be a Post. The simplest organization is to have a single Category named "blog" and put all Posts in there until there is a need to separate this content into definitive categories. Best usage is to have as few categories as possible so readers will be exposed to crossover content.
I keep reading criticisms that author/book websites are not updated enough. The blog area of your website is actually a honeypot to bring search engines and new readers to your book. Rather than silo information into well-separated Categories, you want to have a blog area that crosses many topics related to your book.
Use Tags, not Categories, to sort this information by topic. Your Tags should follow search engine conventions. Pick words that are commonly used over esoteric words specific to your book. Presumably the esoteric words will already be present throughout the text so, for example if you have a blog post with some worldbuilding content about your alien race called the B'nyeagth, resist the urge to tag it "B'nyeagth" because no one will discover your book by accidentally typing that word into a search engine. Use descriptions like "alien, planet, worldbuilding" – these words might not actually appear in the content of the post.
If you don't like graphics don't use graphics, but do create a home page to serve as the dust jacket of your book. Many trendy themes now have a fancy scroll presentation with testimonials and featured content. This fancy presentation only appears on the home page (or at the top of the latest Posts), and this is the one good place for graphics, your "writer's promise" to the reader, etc.
The rest is about tweaking the cosmetic choices of the theme. Don't use a sidebar at least not on your Pages, and find a theme that has a "full-width" template.
I paid for this theme because it unlocked full-width templates that look good for reading.