It's a biography but it's mostly about his history of creation in media. It doesn't focus much on his childhood, and it's all the better for it. Instead, it's broken down by what he was working on and when that happened. There are multiple chapters devoted to some of the longer works, like Buffy, but it covers all of his film, video, TV, and comics up through The Avengers and prepping for Age of Ultron.
The book gives lots more context for his work and it's made me appreciate things I already liked, but in a new light. I have to admit, as soon as I finished it I re-watched Dr. Horrible. It's probably my favorite made for the internet thing ever.
Mostly, it just makes me excited to see what his next project will be. I understand Joss Whedon as a creator so much more now and I'm definitely already interested in all of his future projects. For me, he's now one of those creators that I'll always keep an eye on.