At the beginning of a major project you'll need to spend a full day just doing as much as possible on it. Open up a blank word document and write the title page, a table of contents, some notes for the acknowledgements, the first sentence of a preface. Then just write notes for any aspect of the book that you have ideas for. Put in bibliographical items in the proper format. Basically, you have one document that is your entire book in germinative form. You can return any time, any place to this document and add stuff to it. You will always have a sentence to re-write, or an idea to jot down in rough form. Just always be working on this project.
If you wait until you have abundant time, like a vacation, you will get less done than if you had always been working on it, because the first day of your abundant time will simply be a continuation of ongoing work. That is why I did my jump start on my current project during the busiest time of the semester.