Despite the title, this is neither a Discworld book nor a book about the Discworld. It's a serious, introductory-level history of our own planet and the evolution of life upon it, enlivened from time to time by humorous commentary from the Discworld wizards.
If that sounds like your sort of book, go for it: the authors have made quite a good job of it.
On the way, they generously throw in a bit of everything: astronomy, physics, chemistry, biology, evolution, ecology, statistics, morality - plus Pratchett's chapters in which the wizards do their stuff. This makes for a lively book but one that rather lacks focus. It's not consistently amusing (the science chapters are mostly serious), and yet it's not sufficiently well organized to serve as a work of reference. It reads more like a series of magazine articles.
The two digressions I found most interesting were on morality and statistics. Statistics occupies Chapter 32 and is referenced in the index. Morality comes up in Chapter 6, but you won't find it in the index.