Another Vimes book, and happily much more The Fifth Elephant than Jingo. Another story of Vimes travelling abroad, but this time it's the case that the past is a different country.
Vimes has levelled up impressively over the course of these books, to the point where he's a politician with memories of action. Despite Pratchett's inventiveness, "senior administrator" is a hard role to make an interesting story of. So, instead, this is a most interesting flashback story. But rather make a story about an early part of his career, the plot sends the Duke 30 years back in time, where he ends up as a Sergeant during one of Ankh-Morpork's revolutions. Except it is also a story about his early career, since a young Sam Vimes has just joined the force, and setting him on the right path in a corrupt watch is part of the older Vimes's missions.
At this point Vimes is just some kind of super-hero. From a lesser author this would be... tedious. It fits with the trope of the fantasy series where the protagonist levels up over multiple books and the books become increasingly awkward. In Pratchett's hands, it's still a good story. Vimes becomes a way for Terry to put forward a thesis on good leadership, at least as it comes in policing. Vimes's leadership is backed by working his way up from the bottom, from a good foundation. He has clearly developed hugely since Men at Arms, and his skills allow him to bring the watch house into shape in almost no time - with lots of senior people muttering (suspiciously) that he's clearly beyond Sergeant material.
One thing I instinctively didn't like is the filling in of Vetinari's backstory. We knew he was in the assassin's guild, but not much more about his past and how he ended up in power. It was shocking enough when I learnt his forename, and here a whole pile of history is laid out!
I see this pattern in a bunch of Discworld bits. History coalesces, becomes solid and part of the lore. Early on, they published the Discworld Mapp and Map of Ankh-Morpork, building retrospective continuity on the geography, and filling in the gaps. It feels like there's an ongoing project to do that with the characters. The risk is that building up this level of detail removes some of the feeling of openness to the world (if you don't know things, the unknown is much less well defined!), and can tie down future stories. However, this is balanced by the fact that the Discworld, and particularly Ankh-Morpork, are dynamic. Things change, so there's always something new.
In short, I think he got away with filling in more Vetinari backstory. It was interesting, even if I'd previously enjoyed the mystery.
Pitching it against The Fifth Elephant - I think that book has the edge, thanks to the unusual setting and less well-worn politics. Ankh-Morpork of 30 years ago is still Ankh-Morpork. But it's close, and great fun. Highly recommended.