The initial inspiration comes with the initial idea--which, if it is to be in book form--you better be very inspired by! After that, at least with non-fiction, there is a long research phase, which I always enjoy, which pretty much means reading everything you can find on a subject, taking notes, etc. After that, depending on whether you need to travel to get further information, you start the rather arduous part of writing and writing and writing and writing and then editing and editing and editing and editing and so on. That's the hardest part--or at least the endless self-editing is. For that, I just try and set up a schedule and stick with it. But for me, the editing takes the least inspiration and simply requires sitting down and getting it done.