This is not a novel. It is an overview of certain aspects of actual (more or less: read the book) science, directed against anti-evolutionists, but also against certain misapplications of mathematics and science.
The title is from a book, Natural Theology (1802), by William Paley. He describes a man walking across a heath (uncultivated area). He sees a rock, and may imagine that it has always been there. But if he sees a watch, he knows that watches have not always existed, therefore there must have been a watch Maker. This leads to what has since been tagged "intelligent design."
The authors [and I] do not agree with this. They note two very different uses of the word "theory." One is a misleading equivalence with "hypothesis," which is a question or statement proposed for testing, to see if it can be falsified, that is, proven to be false. The other is an idea about which considerable evidence has accumulated, which has not been falsified. It may not have been nailed down in all particulars, but a heavy preponderance of evidence says that it is true.
The Theory of Evolution is a theory in the latter sense. Certain aspects need further research, but research thus far says it's true, with no scientifically processable information to the contrary.
Roundworld (Earth) was created (in the Discworld universe) by accident, by the wizards of Unseen University, Ankh-Morpork, Discworld. They aren't very impressed with it, but feel some responsibility for its people all the same.
They have recently discovered that one Charles Darwin has written The Wrong Book: Theology of Species, (or The Ology), instead of Origin of Species. This throws a wet blanket over science and technology, with the result that humans fail to escape Earth in time to escape extinction by a new ice age, coming in a few hundred years. They attempt to put human history back on track, fought by the evil Auditors of Reality. Thank Saberhagen's berserkers, crossed with Shakespeare's witches in Macbeth, with a ghost story. This story line is interwoven with discussions of Roundworld (our) math and science.
One of these is the curious idea, held by some physicists, that, because the math works -- is internally consistent and intelligible -- that it Must describe something real. This leads to ideas of time travel, faster than light travel, and multiple universes, generated by extremely trivial pivot events. Not just that Napoleon won at Waterloo, or Hitler won World War II, but that I turned left instead of right out of my front door this morning. Trivial pivots may snowball into large results, but a whole nother universe? And that both exist at once? That both MUST exist? [It's said that, if all the codfish eggs laid grew to adulthood and reproduced, the Atlantic would be solid with cod in 3 years. What would branching universes fill?]
Further threads involve quirks of subatomic particles, and oddities of math. Among these is the googol, 1 followed by 100 zeroes, the googolplex, 1 followed by a googol of zeroes, and, invented by the authors, the umptyplexplex. . . . Well, when you start with a flat world on the backs of 4 elephants, on the back of a turtle, what DID you expect?
But actually the math and science of Roundworld are real and as represented. Recommended.