How to write a book or other very long document

Do so without a word processor. Use some text editor and splash LaTeX in at the end. I assure you that the LaTeX part will suck; I also assure you that it will suck less than Word or OpenOffice destroying your document and you can't even figure out which previous version to use. The versioning step can also be a bit tricky but worth learning to do, particularly with a good program like Versions. As for backing up: I often tell friends who I know don't back up that they can't call me with any tech support questions until they do. That has worked in more cases than I thought it would...