Book designers are typographers by necessity, if not by nature. Content may be king, but content is almost always text. Text must be displayed for a reader, either on the pages of a book, or on a screen.
To display text, you need to use a type font. The font could be chosen by a designer, or it might be a default set up by the engineers who create digital reading devices.
When it comes to fonts, designers have strong feelings. While you’re sitting at a restaurant trying to decide what to order, the book designer at the next table is examining the type fonts used on the menu and how the kerning looks.
I like Volvos, maybe you like Chevys. Same with type fonts. Some designers like Palatino, it does nothing for me. But when you find a font you love, it’s a relationship that can last a lifetime.
For many years my favorite typeface for book design was Bembo. One of the reasons I prize Bembo for book typography is the way it smoothly flows on the page. It’s easy on the eyes even for extended reading, and it seems plain and without any eccentricities.