If writing a book still seems like an insurmountable task, use a timer. Set it for five minutes and just write whatever pops into your head in that time.
Writing against a timer can be a great way to motivate yourself to start writing. If you don’t like what you write, you can delete it, but you may find that some of the material is usable in your book.
While timing your writing doesn’t work for everyone, it’s worth trying at least once. Forcing yourself to write for a few minutes is an excellent way to help you focus.
You can then continue using the timer to write more. Or you can go back to writing without a timer if that works better for you.