Unlike the Player's Handbook, this isn't a book you can sit down and read cover to cover. It's a detailed reference for all the major monsters, creatures, beasts, and beings in the game. It's basically a combination of a world building tool and an encounter building tool for a DM.
I scanned through the whole book, stopping to read about the monsters that most interested me. It was fun! And now I can definitely see how much of a tool this book could be for campaign building.
One of the things I found most interesting was that every monster is given a challenge rating. That rating is basically a "normal" difficulty rating for a party of four characters at that level. So, for example, if a monster has a challenge rating of five then a group of four level five characters should be able to defeat it without dying. It's a great way to gauge the challenge that you're setting up for your adventurers.
I'm glad I have this book to flip through for fun. I think I'm more likely to run a pre-made adventure whenever I start out, but this will be great as soon as I get beyond that.