The #1 justification for authoring any sort of technical tome – and this has been the case for decades – is that it gains you credibility and respect in a field. If you want to consult, or are looking for senior positions, etc, get a book with your name on it out there and you have a foot and a half in the door. At least with many who are suckers and are looking for short-cut filtering, which I say because the vast majority of technical books are terrible.
On the other hand, essays like Philip Greenspun's "The Book Behind the Book" make book publishing seem less prestigious, rather than more (at least in many instances): http://philip.greenspun.com/wtr/dead-trees/story.html , but then he also points out that a lot of the problems come from publishing models more than the authors themselves.
