X2 Power came out 5 months after X2, ie half way through the lifecycle. At this point many users have already worked through the pain of adapting methods and discovering new tools...
While I fully agree with Scott that serious time is needed for a huge book to be written prepped edited and released however to my mind there is no reason for one huge book every time a new Sonar version comes out (X1 excepted) as the differences in workflow and tools is <<10%.
IMO to provide sustainable power (pun intended) and value, this book needs to be out within 2 months of release of X3, and this could be accomplished with a smaller book dealing with the changes.. ie an X3 addendum.
if you have X3 you check in this book first, if its not there then it hasn't changed from X2 and you pick up your X2 book.
regards