I started writing a book as a form of procrastination, and after I had written the first (exceptionally bad) draft, suddenly finishing that book rose to the top of the list. Haven't worked on it since.
I published the open source library I have been privately working on for several months this weekend. It was my way to avoid spending the day closing down my vegetable patch for the winter.
So that is my advice to you: if you want to do the book, add more things to the top of that list :)
Good luck and mention me on the acknowledgements! xD