I get this question a lot. Most people think that designing a book means illustrating or even writing it. I don't write the text, but I do decide how the words look on the page. I don't illustrate in the traditional sense (see: Chris Van Allsburg, Eric Carle, Sophie Blackall, Brett Helquist, &c.), but I do create visual illustrations by drawing or compositing digital images together. I also enlist the expertise of extremely talented illustrators and photographers to help create cover and interior artwork. Design encompasses all of these aspects. It's the glue that aesthetically holds a book together. From the jacket art and paper stock, to the text size and font choice, designing a book is artfully styling its elements so that the theme of the book carries from cover to cover.
How it works
Here's the final cover and full jacket for How I, Nicky Flynn, Finally Get a Life (and a Dog), by Art Corriveau. The book is about the physical and emotional journey of a boy and his newly adopted seeing-eye dog. It's told in a spunky, light-hearted way, so I thought it was important to convey that in the design.
The jacket includes photography, illustrated graphics, and typography that (thankfully!) work together in a playful way. I chose the primary color palette because it makes the story feel vibrant and light. The dog is a stock photograph from Getty Images and I drew the other graphic elements (arrows, sunburst, &c.) in Adobe Illustrator. The title type is PraterBlock One, but it's slightly redrawn to integrate better with the design. All of these elements I carried throughout the interior of the book: