The eighth book in the Discworld series. The first in the Watch sub-series, and so a good starting point. Young Carrot, an adoptive dwarf, is sent off to Ankh-Morkpork to join the Night Watch because they really don't want a six-foot-tall human marrying Minty. Meanwhile, one of the lowest grade secret societies in the city is, under the instigation of their leader, trying to summon a dragon.
You see, this will lead to restoring the king (a convenient candidate is at hand), deposing the extremely politic and austere Patrician, ensuring that people who have carriages are those worthy of them, and removing other injustices. The leader manages to stifle his contempt long enough for them to nudge them into the necessary rituals to summon the noble dragon.
Meanwhile Vimes, the habitually drunk Captain of the Watch, and the other two men find it deeply unsettling to have another man who considers the law something to be enforced and knows it thoroughly (having a book on the subject).
And the Librarian of the Unseen University discovers, dreadfully enough, that a book has been stolen.
What this produces includes the dragon attacking near the watchmen, as if they didn't matter (that they did not, in fact, matter makes it worse for Vimes); obeying orders to destroy tracks but only after taking a plaster cast, a noblewoman who has a refuge for little swamp dragons, the problems with flying when that large, the question of its hoard and its diet, and much more. Lots of allusions to classic Hollywood films, particularly film noir.
Been a while since I read it. I was thinking that he had gotten a good grip on Discworld here, and indeed, very little world-building in this gets revised later. OTOH, what are the targets of his parody changed a lot over the series. It changes the mood and the tone noticeably.