Design. Experiment with several different cover designs, page layouts and typefaces in careful consultation with potential readers. Expect this to go through several rounds of revisions, making sure you get a look and feel you're happy with. Use some existing random text for these tests.
Decide how many words the book will contain.
Decide what the book will be about.
Write the book.
It's a strange approach, isn't it? But websites are developed this way all the time, with the actual content regarded as a last-minute detail to be filled in at the end rather than as the core of the project. To me, it's always a warning sign when the copy is referred to in project meetings as "text" -- as though the important thing were what it looked like, rather than what it said.
Copy is not just another design element.It's often said that good design is unobtrusive. Rather than drawing attention to itself, it should serve the content of the site, making it easy to use and navigate. For most sites, that content is still in the form of written words. The first goal of the design should not be to look nice, but to help users read the content; design for design's sake is a mistake.
Of course, there are shades of grey: reversing the process above and leaving all the design work to the last minute would be a mistake, too, but that is a much less common problem -- in fact I'm not sure I've ever encountered it. That's why we always encourage our clients to think about copy content as early as possible in a web development project: it's our experience that when form (design) follows function (content) you have a better result.