In 2004, Dan Gillmor wrote "We the Media", one of the first books to illuminate the role of "citizen journalists". "Mediactive" should be seen as a follow-on to "We the Media", not an update or sequel. It's split into three major sections: Media literacy, Media creation, and, for lack of a better term, "Other". The first section is essential reading for anyone, but it would be particularly valuable for high school and college students who are still learning how to judge whether a story is valid or an information source is reliable. The second section starts with a high-level overview of citizen journalism and dives into the nuts & bolts of blogging, Twitter and other means of self-publishing. This section is written primarily for novice citizen journalists; if you've been doing it for a while, you already know most of what Gillmor says.
The third section is, in my opinion, the least useful part of the book. It starts with a review of the laws and norms governing journalism and communications in the U.S., but it's at best an overview of the issues that any journalist should be aware of. Then, it jumps into a discussion of teaching and learning media literacy and journalism, and concludes with what needs to change in order to improve the practice of journalism. As Gillmor himself admits, the latter topic alone deserves its own book, and that's the problem with the final section of "Mediactive". Each of the last three chapters would work much better as separate books, and there are books that you can buy on the three subject areas that are much more comprehensive than the information that Gillmor gives in a single chapter.
In short, I strongly recommend "Mediactive" if you or someone you know is serious about developing media literacy skills, and moderately recommend it if you're interested in citizen journalism. If the topics in the last three chapters are of serious interest, you'd do better by buying books dedicated to the topics.