This book project will entail a number of important aspects, and its success will be determined by them:
1) Writing the bulk of the content, which will be derived not only from the various Synopses, but the official test suite and directly from the active developers. Information which is speculative (such as from non-finalized synopses) may be included but will not be the focus of initial effort due to their unstable nature. A successful book will include the information and examples necessary to teach the core language to a person who is not currently familiar with it, and bring that person up to a basic level of understanding.
2) Aggregating and integrating existing Perl 6 documentation which is licensed compatibly with Wikibooks, or for which the authors agree to grant a license for the use of their work at Wikibooks. While not a metric for the success of this project, being able to include a comprehensive listing of related resources and obtain small donations of content will help to make the book viable and to get a larger community involved.
3) Attracting readership and additional authors/editors/contributors to the book to ensure it remains viable in the long-term as an authoritative Perl 6 resource. Success in this is harder to gauge or estimate, but the net result should be that more readers are exposed to Perl 6 and able to learn it on their own, and that more volunteers are attracted to the Perl 6 project to aide in documentation and book writing.
Good luck Andrew! If you're interested in submitting a Perl 6 microgrant, you can at find more information about how to do so.