I know I read the first Science of Discworld book, and liked it. I thought I bought it for the club, but I'm not seeing a SFBC edition on ISFDB, or any edition in the US earlier than the trade-paperback series starting in 2014, so perhaps my memory is faulty. I also read the second one, and thought it was not quite as strong, and I think got to this one around UK publication in 2005 (thanks to Colin) and had a similar reaction.
There was a fourth book; I'm pretty sure I've never seen that one.
All of the Science of Discworld books are hybrid things. There's a fairly slim Discworld fictional story by Pratchett - about the creation by the wizards of Unseen University of what they call "Roundworld" (our universe) and their subsequent investigations and adventures there. In alternating chapters, scientists and science writers [2] Ian Stewart (math) and Jack Cohen (biology) explain the real science behind the goofy stuff the wizards get up to. I expect the three plotted out the whole thing together and then edited each other for consistency; it's a bit of an odd combination but it generally works on both a comic-fiction and a pop-science level.
So the first book saw the Roundworld created. The second one had the wizards needing to save it from an infestation of Discworld elves, mostly related to Shakespeare for sufficiently-good reasons.
And this third book, The Science of Discworld III: Darwin's Watch, came out in 2005 in the UK but not until a decade later in the US. It's the one about evolution, so I'm afraid Stewart and Cohen need to talk about the kind of anti-evolutionists they characterize as Christian fundamentalists from the Southern USA. (Not inappropriately, of course, and entirely honestly. But it might have been yet one more thing to give US publishers pause.)
Once again, outside forces - it's not revealed until late in the book, so I won't say who, but it's another Pratchett villain - are interfering with the Roundworld's timeline, causing humanity to go extinct before it can lead a space exodus to colonize the larger Roundworld universe as of course humanity is destined to do. (Science of Discworld is more old-fashioned than some readers might expect.) This time, it's all connected to Charles Darwin, who is being channeled to instead write The Theology of Species rather than The Origin and to stay a good Anglican curate and not be so strident about the whole evolution thing.
The pop-science chapters are an odd mix of evolutionary biology (not quite as much as you might expect) and QED-derived alternate-world physics theorizing circa the early 2000s (because the changes to Darwin's timeline creates many different options, in none of which he wrote Origin). There's some other, even further afield science topics as well, but those are the two central ones: the ways species change and diverge over time, and the ways timelines might conceivably do the same. (They are connected, at least conceptually, but they come across as mostly-separate discussions.)
The short Pratchett chapters feature wizards - mostly Mustrum Ridcully, Ponder Stibbons, and of course Rincewind - having somewhat amusing experiences either watching Roundworld or traveling to it to interfere with its timeline, and talking about the science implications. Then the longer Steward/Cohen chapters go into more depth, non-fictionally, about those implications. It's all amusing, mostly light, and easy to follow: it's a fine pop-science book, if possibly slightly outdated now, twenty years later.
There's enough Discworld that you could read nothing else for quite a long time if you wanted to, and I suppose this series means that those who do - I'm fairly sure there are some anorak-wearing chaps named Kevin who come close, at least - get some actual science knowledge included into their humanistic humorous fiction. I found this one less focused than I'd hoped, but all of the pieces are solid, and they form an essentially coherent whole, so I'm mostly quibbling.
[1] This is complicated and changed over time. But, very generally, in the '80s and '90s, Pratchett was not just the biggest fantasy author in the UK, he was one of the top five or so authors of any kind in the UK, and a measurable fraction of all books sold had his name on them. That profile declined only slightly post-Rowling; I think he still sold basically as well, but others sold at similar or nearly-similar levels, too. Because of that, he could write or commission the kind of oddball line-extension books that were just not commercially feasible for almost anyone else: a cookbook, a long series of annual desk diaries, multiple actual printed-on-heavy-stock maps, an encyclopedic compendium in several editions.
In the US, by contrast, Pratchett sold like a decent upper-mid-level fantasy writer (comparing it to personal income, in the US Pratchett was like the well-respected name partner of a major law firm and in the UK he was Bill Gates) and was beloved in the SFF community. It was absolutely a solid level of success that most writers would be thrilled with, but Pratchett (and, more centrally, I think, Colin) saw the UK sales and thought "why can't we replicate that in the much larger and more lucrative USA?"
There's also the question of how well he was published in the US. I don't want to cast aspersions, but he was with Roc/New American Library at first, who perhaps didn't have the market muscle (or the desire to use it for Pratchett) to replicate the UK success. In the early '90s, he moved to Harper, which went through a confusing period - Pratchett was very prolific at that point, and Roc had gotten behind - where the books were not published in the same order in the US and there were some interesting cover designs in attempts to widen the audience. So, at the point where he was at his absolute peak of market power in the UK - say the late '90s - he was rising in the US after Harper got good at publishing his books but was still very much a genre author rather than a general-audience one. That has stayed true in the US since then.
And, frankly, Pratchett is deeply British in a lot of ways. There's no reason to believe he would be a general-audience author anywhere that doesn't have a solid British cultural identity.
[2] And science-fiction novelists; I read and bought their Wheelers - not for nepotistic reasons related to the title, I think - for the SFBC.


















