DEVELOP A WIREFRAME
A wireframe is how the book is organized. It consists of the number of pages, book title and subtitle, chapters, references, appendicies, foreward, preface, acknowledgements and introduction. Start with the number of pages. Many books are 300-400 pages. For example say you want to write a 400 page book with 15 chapters. When you do the math that is about 26 pages per chapter which may be too much. If you choose 20 chapters, the number drops down to 20 pages per chapter which is more reasonable. Now develop the number of words on a page. If you use 10 words per line and 35 lines per page, that's 350 words per page. 350 words per page times 20 pages gives you 7000 words per chapter. Wordprocessors contain word counts so just write each chapter and pay attention to the word count until you reach your planned target for the chapter.