Honestly? Primarily non-economic reasons. (People asked me for it, the Hyperink guys seemed great, "published author" beats "owns a software company" in the McKenzie Family List of Approved Jobs, etc.)

It is possible that there are some markets and consultancies for which publishing a book would be a great credibility enhancer, but I sort of doubt I'm in one of them. I do something which is very quantifiable, with very few people available who do it, and which empirically produces absolutely stupid ROI some of the time. Books are not required to raise rates. (And, if I ran into a company that was unpersuaded by "I made $FIRM $X million in two weeks by running a series of A/B tests" but really responded to "... and I wrote a book!", I'd run screaming from that engagement, because that client sounds like they're going to be a lot more stress than my savvier clients.)

He charges $X0,000 per week for his consulting services and has had more than modest success in doing outreach/education/marketing through his blog and participation here and on the SEOmoz and Business of Software boards. He's past the point where writing a book is the most remunerative use of his time.

Jealous??