Prince and the PMI assume a default life cycle of waterfall. You don't have to use waterfall to do what they say, but their assumption is that first you'll get the requirements, and go from there. My experience (and therefore _Manage It!_) says, uh-uh. If you can't get the requirements up front, you have to manage the project (because of the risks) differently.
Instead of whining that I can't get the requirements, I learned to manage projects differently years ago, which is why I wrote the book. If you're in an organization wedded to a Big Name Methodology, Manage It! can help you see other options that might make sense and help you steer the project to success.