I don't know if this is actually going to help me write a book but it certainly is a great post. (I found it through the Mac Power Users podcast.)

Seeing the care you obviously put into your prose, I feel I should point out the(?) remaining typo:

"you’re begin to tie up"

I'd also second the Scrivener suggestion. It has far more tools than I need, but there are two that make a huge difference. First, the binder works well as a container for lots of bits of writing. So I make a folder for each planned section or chapter, each of which can contain lots of bits of writing, which can be reordered at any time and viewed separately or combined. I also have a folder for notes that are relevant to the project but I don't know where to put. I find this way better than square-bracketed notes to myself, because each note can be given a name that is viewable in the binder, ready to be moved later on to somewhere where it fits. There's also another folder for quotes from other works that I want to refer to, along with the bibliographical information. All in all, the binder makes it much easier to start writing anywhere in the project and then incorporate the writing into the structure later on.

The other feature that makes writing easier is the split editor. That allows me to refer to external sources or to earlier rough notes of my own while writing something more polished. It occurs to me that it would also be ideally suited to your tip to retype a few paragraphs you wrote previously to build momentum.