I'm in the middle of reading this book and thoroughly enjoying it.
If you've not read any of the Science of Discworld books before, they are a series of books that are fairly easy to misinterpret the contents of from the cover (unlike for example, an issue of Heat magazine, where you know exactly what's in it from the cover).
Far from being a fictional science trying to explain how a giant disc floating on four elephants and a turtle can function, it instead approaches modern science in our universe.
Alternate chapters are a fictional setting where the wizards of Unseen University have accidentally created our universe in their High Energy Magic lab and are trying to figure out how a universe with no Discworld like gods, giant turtles or magic can function (and still produce humans). They then occasionally intervene to make sure certain events happen. For example, they make sure Charles Darwin writes The Origin of Species in this particular outing.
The chapters in between are a serious discussion of the real science behind what the wizards are observing, covering concepts as evolution, the mathematics of infinity, whether a time machine is mathematically possible (more importantly, is it physically feasible?), how various indirect evidence has led us to our current understanding of the age of the earth and universe, the various theories of the many worlds multiverse, etc. I find this kind of thing really interesting.
They also take a fairly dim view of religion (like myself), ripping creationists to shreds (if people cannot believe the mountains of evidence in front of them, why do they place all their faith in a single inconsistent book?).
In a way, the books are similar to Bill Bryson's A Short History of Nearly Everything
which covers a lot of similar scientific ground aimed at a general audience. However, I feel the Science of Discworld books are a better read - mainly because the scientific understanding and narrative are deeper and the authors try to make serious points in order to make you think differently. Also, they make the point that despite the wizards making obvious errors in their understanding of our universe, these are the same misunderstandings that past (and current) scientists have made.