Congratulations on the new book. GEMs are a compelling part of the Ruby infrastructure, in my opinion.
Your book fills a vacuum, in that the documentation at the formal Rubygems site is (or at least was when I last looked earlier this year) incomplete and sometime mutually conflicting or related to earlier releases. Was this a 'challenge' in getting authoritative information on which to base the text of the book?
Apart from "why don't you do it", do you have any suggestions for getting the core rubygems documenation improved?