So far the challenge has been to feel like I've accomplished something, or have some metric for accomplishment. I've done a little writing, some revising, and a lot of playing around with Visual Studio 2005. If I had to guess, I'd say I've spent about 30 hours each week doing these things. I just don't have a lot of physical stuff to show for it (aside from experimental code).
I've also got a client project I need to finish up. I'd say the thing that's different about not working for The Man is that you tend to work when the motivation strikes. In that respect, I put in lots of time, but not in four-hour blocks before 5 p.m., as you would in a day job. You really need to look at things differently or you might think you're not really doing anything. It might appear that way to your spouse, too! ;)
Regardless of what might come of the book or the future, doing this has been liberating, and an amazing experience so far. Anyone not happy in the IT industry needs to give it a shot. The worst you can do is go back to work for The Man.