How do I convert my PhD dissertation so that it can be published as a book?
Why do you want the book? 


For prestige?: publishers who contact you, don't do any review and charge high prices will only increase your prestige to those that don't know them (and these are probably not those you want to impress).
For dissemination?: why not just put the pdf available (as was already recommended)?. Those who are interested will find it, those who are not will probably not buy a PhD thesis
For money?: I doubt you will really make much out of it.


My recommendation: put it as a pdf on your website / your school's dissertation repository. If you see that lot's of people download it, consider an improved version as book.