Discworld/Men At Arms/WMG
Vetinari has something of a Heel Face Turn in this novel, motivated by Vimes and Carrot.
Think about it: the Patrician completely failed to read and manipulate the situation correctly. He tried to push Vimes into solving a series of murders that he was already working on, and that attempt actually drove Vimes into retirement. He had no idea that Cruces was the murderer. And at the end, Carrot's proposal to rebuild the Watch takes him completely by surprise.
What we're seeing here is Vetinari realizing that his mental model of Ankh-Morpork and of human nature in general is inaccurate. That there are not "always, and only, the bad people." That it is possible to improve the city while preserving civic order, not merely leave it balanced in its misery.
It is only after this novel that Vetinari's manipulative skills really begin to match his towering reputation for having them.
- By extension, if this is the book where he had a Heel Face Turn, then Night Watch is the book that'd convinced him to become so cynical in the first place. He'd seen his own aunt's meticulous efforts to replace a corrupt and paranoid Patrician end up merely putting another ruthless backstabber into power. He'd applied all of his hard-won, elegant expertise from the Assassins' school to eliminating Winder, only to see Snapcase turn around and sic an uncultivated psychopath on the only sincerely well-meaning unit in the Watch. Vetinari's belief that the good people are only of use to replace one bunch of bad people with another, we can now see, stems from direct personal experience from that period, and he only shakes off that disillusioned cynicism when Carrot's proposal for a new Watch—and the could-have-been-king's complete disinterest in personal power—shoved his face in the fact that not everyone he might trust to share the civic authority is a Snapcase-in-waiting.
Detritus' large equation has an obvious answer
When he gets locked in the pork futures warehouse, he creates a huge, elaborate "theory of everything" equation all over the walls, in an attempt to pass the time until he is rescued. He gets everything down to one last "=" before being rescued. I suspect that the answer on the other side of that equals is 42. The Discworld Universe has the Question, and the HHGG universe has the answer. Unfortunately, because this universe has seen the answer, we'll never find out the details of Detritus' question.
The Lord Rust mentioned in this book is not the same one as the Lord Rust in later books.
He could have died between Men at Arms and Jingo whereupon the Lord Rust we all know took over.
- Jossed as of Snuff, unless something very complicated happened to the succession, because the Lord Rust in that book is notably older than Vimes.
- In addition, Lord Rust makes an appearance in the past during Night Watch, and Vimes' reaction to him confirms that it is the same Rust as in Jingo. Once again, unless the family succession got very messy, it's more likely a case of Characterization Marches On, as with the Patrician in The Colour of Magic, according to Word of God.