Richard Hendel says:

Designers are to books what architects are to buildings. Designers write specifications for making books just as architects write them for constructing buildings. Even the most seemingly mundane detail needs to be decided, and it is just these tiny particulars that make a design successful. The parts of the book that are most ignored by readers are the ones that often need the most attention from the designer . . . The author’s words are the heart of book design. To solve the design problem for a book, a designer needs to know both what an author is saying (what a book is about) and how it is being said (the actual words being used).
On Book Design by Richard Hendel, p. 33

When I read Hendel’s words, I am in total agreement and I want to add that the relationship between the designer and the author is important. Not only is the designer working with the nuts and bolts of picas and points and margins and folios (I’ll provide a glossary on this blog soon), but the designer is also interpreting the author’s words visually on the page.

So the self-publishing author and designer must have a rapport, an ability to communicate and understand each other.