Part of the difficulty I'm having is that for large chunks of this book, I find myself once again completely rewriting everything, since for large chunks of it I haven't actually touched it since about three or four years ago. And I don't know about anyone else, but I can't write a terrific draft on the first go, and so not only am I editing the old draft (by rewriting it), but then I have to go back and edit the new draft as well with the same kind of care and patience.
I was also stressing for a bit these past few weeks, because up until last week I assumed I would need to be getting into the job market. The original plan was for me to finish up my PhD next year, which meant I had to start applying for postdoc positions this month or in October (they always advertise a year ahead of time). But much to my great relief, my adviser sat me down and informed me that he thought it a better idea I take another year to finish, since the job market right now is super competitive, and I would be that much more competitive next year with a few more papers under my belt, some observing proposals, some more intelligent things to say about our H_alpha images, and so on. So while these past few weeks I've only been able to scrape together something like ~30 minutes each day to sit down and write, now I should be able to go back to my usual ~1-2 hour a day schedule.
I'm about 1/3 of the way there.