That's awesome, can I ask what the topic is? What I did for "selling" the book was to create a proposal -- about 45 pages that has a skeleton outline of each chapter (it changed significantly during the end writing process but gave the publisher a feel for the topic), a sample chapter, and some more sales/marketing details like what are comparable books (and how well they sold if you have that data), who your audience is and how you plan to reach them (OP was in a great place having a following), why you're the right person to write the book, etc.