If you are wanting to read the "back story" behind the music and are just now starting your homework, let me suggest you start here. Why? Why here, when this is obviously a flawed, overly subjective work seen through a prism of chemical distortions, bringing us what are probably broken and incorrectly reassembled memories? Because this is a book you will finish. You will read this from cover to cover and most likely love it, and because this book is (more than any other out there) about the FUN of the Grateful Dead. That part gets left out - a lot.
Other reviewers are not wrong - the last half of this book is largely about Scully and Garcia's drug addiction. But it isn't, as is made clear, like everyone else was a health food nut. (Well, Bobby was, but that's beside the point.) And there is also a ton of history going on during this time, too. (For one thing, we learn some of the reasons that Bob Dylan was so devoted to Jerry and said such gracious things about him later.) But what made it all work, the glue that held it together, was the fact that this music was just so much more fun than anything else going on. This book is about that fun, and this book is fun to read. There aren't many books that have made me laugh harder.
Where you go after this is your own business: if you want to read a superb biography and perhaps the most important book of the whole genre, read the Garcia biography. "Dark Star" is heartbreaking but very insightful, and much of it makes "Living With The Dead" seem tame by comparison, as it is all first person interviews of persons involved. The McNally book is probably the completest, but is often as dry as toast and completely disengaged from the joy this band dispensed. So start here for fun, and to get a taste for what the life was like, and put a little color in the cheeks of all those black and white photographs.
And as to why this book doesn't get much into the music, it's because no book could get in to the music and talk about anthing else. Scully was not a Dead head - he would probably rather have seen a Stones concert any night. He worked for the band, he didn't follow them for love of the music. If you want to get inside the actual music, that's a whole separate library you need to read. We aren't talking about the songs, we're talking about the band, and this is as good a place as any to meet them, and better than most.