Totally agree about not obsessing over tools. That's an easy rabbit hole to go down and distract yourself from producing the actual content of the book. I deliberately chose to use Google Docs because the formatting options are so limited. But I also found it motivating to have a WYSIWYG interface so it felt like I was looking at something resembling the finished product.

In the end gdocs turned out to be a great choice because it's so easy to collaborate. All of the articles talking about markdown seem to assume that you're the only person that's going to work on it, or that everyone else is going to figure out this diagram and your git workflow. It's so easy to email someone a link and let them comment on it, or suggest edits.

For producing the final PDF I found a plugin that embeds the doc directly from Google into InDesign so I can typeset it into something that looks pretty great. Then I just upload the PDF to Leanpub.

The tool that had the biggest impact though was Beeminder, which has a hook into Docs and measures your progress by counting the number of words in the book every day. If you don't make progress you have to pay! It's a great way to make yourself accountable. I kept a 100+ day streak for the final push.