It's more complicated than that. The study referenced in the book found that snake_case makes variables easier to read, but the whole program harder to parse (my guess is it's because of the visual similarity between the underscore and a space). With camelCase, it's the other way around.
I vote for kebab-case, but I didn't find any study where it was included...
I vote for kebab-case, but I didn't find any study where it was included...