Roughly 2/3rd sourcebook and 1/3rd adventure/campaign, this book does a good job of establishing the core setting for the 3rd edition of Mutants & Masterminds.

The sourcebook section is solid. The first major part consists of public knowledge that anyone either knows or could easily find out about the city: history, landmarks, neighborhoods, public services, etc. The second major part is the secrets section. The hidden details that the heroes are unlikely to know about, at least at first. There is a bit of inconsistency of style in this section, as some of the write-ups include a brief summary of whatever it is that is being discussed, while others just describe the "secret" information, sometimes requiring a flip back to the earlier sections to get a refresher on just what subject is being talked about. It's a minor quibble though, and overall the information in here is interesting and useful.

Also included are stats and write-ups of the iconic pre-generated heroes of M&M 3rd Edition, and a section on using Emerald City as your campaign setting, including adapting the material provided to different default assumptions, such as setting it in a world where heroes are a new thing rather than an established reality.

The adventure section is a bit more hit and miss than the sourcebook. It gets off to a rocky start by directly contradicting the sourcebook on the identity of the police commissioner. There's a box that supposedly corrects the problem by saying the commissioner in the adventure retired shortly after the adventure (which takes place six months before the information in the sourcebook is considered to be current), but it's heavily implied that the commissioner in the sourcebook took office around the same time as the current mayor, and she's been mayor for over 15 years!

A bigger problem with the adventure is that it's designed for 6 to 8 players, which I think is larger than most groups are going to be able to put together. Still, there are some good ideas scattered throughout, especially nearer the end of the adventure. Well worth adapting to your own scenarios even if you don't use the adventure as written.

Overall, a good book, and a good setting if you want to set your superhero games outside of Marvel or DC, but don't want to come up with a complete setting all on your own. Maybe not so good if you want to have a small campaign to run out of the box (unless you have 6 to 8 players in your group), but you'll still probably find something useful if you plan on running some Mutants & Masterminds.