First thought that came to my mind when reviewing the new book from David Mercer on Drupal 7 was "Its too early". As in, it seems to early to be releasing a Drupal 7 book considering that Drupal 7 hasn't been released. I understand that the Drupal community want to launch Drupal 7 and hit the ground running as previous releases have been slow to pick up momentum but as this particular book targets users who will use and administer Drupal 7 from the GUI interface, I am cautious to recommend the first edition.
Flicking through the first few pages you immediately find the what is likely to be the biggest flaw of the book, there are screenshots and instructions on how to use Drupal.org that are somewhat confusing now that Drupal.org has gone through a complete redesign and the Drupal.org landing page and high level information architecture is different now. While there are no major visual or interface changes expected to happen before release, the possibility does exist, especially with contributed modules for Drupal 7.
After that rant I should turn it around however and point out that the major improvement to Drupal 7 above all else is that Drupal 7 is be improved to be more intuitive and user-friendly as previous versions had a large learning curve to understand the administration interface. This book by David Mercer compliments Drupal 7's workflow very well. Reading this book, I did get the sense of how Drupal 7 worked and a quick confidence in using Drupal 7. It also highlights major improvements to previous versions which standout as selling points for Drupal 7 compared to other CMS's around.
Without a doubt, I think this book is needed for Drupal 7. If you no nothing about Drupal or want to be ready to use Drupal 7, this book will be good for you. Just wait for the second edition with the Drupal.org updates if you think you might have trouble improvising when getting started.