This is the eighth Discworld novel. Imagine, only a few books in - and the characters are still developing! It's the first, I think, in which the Patrician, Lord Vetinari, is more or less the Machiavellian figure we love, pulling everyone's strings. He has arranged the Guilds to be so busy fighting each other over honours and precedence they don't have time to unite against him. He has mimes thrown into the scorpion pit with, "Learn The Words" written on the wall. When thrown into his own dungeons(designed to enable him to escape if he's ever locked in) he soon makes himself comfortable by advising the rats in their fight against the scorpions, and then having them serve him. The Guild of Thieves is responsible for making sure crime is organised, thus limiting the levels to licenced theft.
Of course, that means he has had to arrange for the last of the police force, the City Watch, to be struggling and irrelevant, led by the drunken Captain Vimes.
Yes! The first City Watch novel! Sam Vimes appears in the first scene, drunk, in the gutter, still grieving after the funeral of one of his last men. If you've read the series, you know that this proletarian hero will marry a rich and kindhearted woman, have a little son and be dragged kicking and screaming into the aristocracy. Eventually. Meanwhile, it's fascinating to see him right at the start, with nobody but his comrades Nobby and Colon, who have learned how to survive in the Watch, and their new young member, Carrot, the adopted dwarf who is six foot six in height and probably - no, certainly - the long lost heir to the throne of Ankh-Morpork.
You know that this boy will one day be Captain Carrot, who knows the city like the back of his hand and seems to know everyone in it. He is still naive, but already able to shame a pub full of roistering Dwarfs who, in his view, should be living quietly and writing home to their mothers. And, by the end of the book, able to get the hardened cops to follow him.
Vimes is very much as we know he will be, though lacking the confidence of later books. But he is the Vimes who is proud of the fact that the citizens of Ankh-Morpork will treat any public dramas as street theatre and try to sell you something while the crowds gather. He is the Vimes who won't let something suspicious go uninvestigated, the man in later books known as "Vetinari's terrier."
It is, I think, the first novel to feature Cut-Me-Own-Throat Dibbler, vendor of anything he thinks the crowds will buy, but basically sausage-in-a-bun. He is a very good salesman; people who have eaten his dreadful wares before will still buy them again. He is pretty much as we know him from later books, though he will be seen having a go as a film producer in Moving Pictures(he only sells sausages when some big get-rich-quick scheme has failed). In that novel, we will see just how over-the-top crazy he can be when he gets hold of some power, but right now he's just the man with the tray of sausages(and dragon-dolls and anti-dragon cream...whatever will sell).
Detritus the troll, later to be a good, reliable member of the Watch, appears briefly as hired muscle(a splatter rather than a bouncer) at the Mended Drum pub.
The Librarian has appeared before, but this is the Librarian we know well from later books, the ape who definitely doesn't appreciate the M word - and it is the first mention of L-space, a place which only senior librarians who have performed valiant deeds of librarianship, know about. It's the reason for all those second-hand bookshop proprietors who seem to be aliens.
There are minor characters who will only be mentioned in later books, but when I recently reread The Truth, a mention of the Dowager Duchess of Quirm made me think, "Ah ha! Brenda Rodley, friend of Lady Sybil, fellow breeder of swamp dragons."
It's exciting to see it all at the beginning and know how much more there will be and how much you'll love these characters in future books, as they grow and develop. Next stop: Men At Arms!