I have to admit, I'm not an Authority fan. I'm not a fan of Wildstorm, never read a book until recently, only heard about the cartoons involving the WildC.A.T.S., et cetera. I wanted this book solely on two merits - I wanted to see the new powers in the Silver Age Sentinels system (d20 version) and I wanted to see how they defined specific powers so I could parallel them in my campaigns.
All that and a bag of chips.
The Hardcover is a nice little book with a recap of the first 12 issues, plus the "Secret History" of the Authority. It details the main villains of the first three stories, and gives a goodly amount of campaign data. It also contains full d12 rules (SAS standard, I believe) for playing the Authority and original characters in said universe (and neighboring ones, as well!), some new (specific) equipment and, before the d20 updates, a wonderful little bit on the philosophies and storytelling of the modern age of superheroics.
While the book is a little light on a "what's next" role (only providing statistics for the 12+ issues it covers, really), it does have a lot of detail on what's been done. Perhaps its greatest strength is the demonstration of how to stat out powers for use with Dynamic Powers and Power Flux in the examples of the Engineer and the Doctor, as well as proposing a way to do it with othe such heroes. This is a tremendous resource, IMO.
Overall, this book is worth the purchase, whether as a supplement or as a roleplaying system. It has all the mechanics and rules to run a full game, with no other books. It also has a fascinating look at the "modern" age in terms of costumes and morality. For this, it's 4 stars. A fifth star would've been awarded for an idea of follow-up (how do you really follow up a nation of super-clones, a war between dimensions, and destroying an elder (Cthulian-style) God?), but I would (if I could) give a half-star for its ideas on adding members to the Pantheon of the Authority.