My question about books like this is always 'If you are doing so well at what you're writing a book about, why are you wasting the time writing a book about it?'
Surely you don't expect that your time spent writing a book is going to pay out at $300 an hour, or whatever your consulting rates are.
I think bdunn and robbiea's suggestions are valid.
(And, man, I wish pyre's suggestion that I don't need money was true :o) though I'm lucky enough to consider myself pretty successful at what I do, so I wasn't writing to make up for missing revenue.)
But, in addition to the residual income and, to a lesser extent, publicity angles, the initial spark behind writing it was the fact that I found myself being asked time and again about what life is like as a contractor/freelancer. Which meant that a) there was definitely a knowledge gap/problem to be solved and b) the repeated nature of similar requests meant it made sense to optimise how I shared what I'd learned over the years.
I'm well aware that no one gets rich from writing a book - so I deliberately didn't sacrifice any client time; I was able to (just) fit it into what spare time I had.
Along with all that, there's the fact that I knew I could write this book and wasn't intimidated by the task (I had the knowledge and the skills -- I used to be a national-level journo) and the fact that writing is still fun, so it was a pleasure to make.
"I found myself being asked time and again about what life is like as a contractor/freelancer."
This was the same impetus behind indieconf, the conference I run in November (shameless plug - http://indieconf.com - we might even get bdunn speaking this year).
Hey Steve, I'm running a startup focused on making freelance dev easier, http://dragonflylist.com. Love to have a chat sometime on skype: riley.james.aus
Honestly, if you're really able to pull $300 an hour, you run out of things to do? Work more, donate the money to charity. You can easily save a life for $300.
Surely you don't expect that your time spent writing a book is going to pay out at $300 an hour, or whatever your consulting rates are.