For another view on this see William Germano, From Dissertation to Book. http://www.press.uchicago.edu/Misc/Chicago/288463.html The author says, and I agree, that a dissertation can be turned into a book successfully.
Some differences between a dissertation and a book: topic (narrow vs broader), purpose (demonstrate knowledge to superiors vs inform learners), audience (faculty vs students & nonacademics), style (purely academic vs appealing, readable, comprehensible), and author authority (frequent citation of previous works for credibility vs creating something new from a position of your own authority). Academic presses don't make a lot of money from book sales, and they don't want to lose money on your book, so sales are important. The way to increase sales is to have a wide audience, which implies a broader topic and a different style.
If you want a career in academia, your first impulse may be to squeeze several published papers out of your dissertation. But if you're constantly recycling your dissertation into a series of articles and a book, your growth as a scholar will be limited. The lack of new topics will stand out on your CV when you're applying for jobs.