Chris Anderson of Microsoft concludes in weblog that writing a book is a lose-lose proposition. Towards this he gives a number of plausible reasons to write a book, analyses them and arrives at the conclusion that writing a book is not worth the effort.
I disagree. He doesn't list one item that was the driving force for me to author my only book. And that is: Self-education.
I don't mean to say that one should write a book to learn about a topic. But if you like and care about a topic and want to do some advanced research around it, then writing a book is a perfect way to do so. There may be others -- like reading good books or getting involved in a development project or joining a formal education/research program -- but nothing is as satisfying and cost effective as signing up to write a book.
Added with other worth-while objectives (and the weight for each objective could differ from inidividual to individual), Writing a book could be a a justifiable commitment.