Working with Word is okay, but there were a lot of early formatting errors when I uploaded it to the publisher. Also, Word has some serious limitations when you're writing a very long document. I bought Scrivener, a program designed specifically for writers, to work on Book 2. There are a couple of things I find vaguely annoying, but I've only had it a month now, and haven't learned everything yet, or broken my old Word habits.
Even with my twenty-plus years' experience with Word, I'm finding Scrivener much easier when dealing with this length of material. I will soon find out how well its exporting features work. I had some issues getting the beta version to convert, but it was my first attempt, and I found an easy work-around. I still use Word for shorter things, like this blog.
When reading reviews of the program, they kept mentioning the cost. That had me worried it would be super expensive. It's not. Plus, there's a free 30-day trial, so you can see if it works for you before you buy it. And that's 30 days of using it, not 30 days from download (so if you don't write every day, the actual expiration date is longer).