I recommend "What I talk about when I talk about running" by Haruki Murakami.
Writing a book or whatever is like running a marathon. You just got to keep at it everyday. Set yourself time for it. Sit down, and even if you are not inspired or motivated just sit there. You keep digging and digging until you strike a reservoir of inspiration. Relying on motivation is not a good idea for wirting. Motivation comes and goes. Discipline is key. Whether it is setting a word count target or just spending time working on it whether you achieve something or not.