As you can see below, there are a large number of crossover mentions in this book. This is probably the first of the really three dimensional stories of Ankh-Morpork.
Wyrd Sisters tells a little about the Fools' Guild and
Pyramids tells some about the Assassin's Guild, but
Reaper Man brings all of the city together in the Oblong Office of the Patrician. There is even an allusion made to werewolves coming from somewhere int he mountains, and we're many books away from
The Fifth Elephant and a visit to Uberwald.
And really, you could make the case that everything that happens with Windle Poons and the city of Ankh-Morpork are all background and subplot to the real story of Death and Bill Door instead of them being parallel stories.
And from listing everything, I can see why this used to be my favorite of the books. It seems like almost everything is in it. There are new major players introduced, and only a few who aren't.
And it reads like an action movie. There are two pivotal fights going on very from away from each other, but at nearly the same time. And Pratchett cuts back and forth between the two like you would see in a movie. Many of the Discworld books seem at a reading to be easy to make into a movie, but I think that is the strength of his imagery rather than any easy transition to a script.
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Appearing in this book
“Sixpence is sixpence.”
!!!!! - the sign of an insane mind
Ankh-Morpork
Arthur and Doreen Winkins, The Count and Countess
Notfaroutoe
Auditors of Reality
Binky
Blind Io
Brother Ridcully, Priest of Blind Io
Casanunder the Dwarf
Chalky the Troll
Continent of XXXX and the Lost Wizard Colony
Cripple Mr. Onion
Cut-me-Own-Throat Dibbler
Death
Death's Daughter
Elm Street
Gimlet the Dwarf and his Delicatessen on Cable
Street
Golems
Hoki the Jokester
Hygiene
Ixiolite, the stuttering banshee
Lancre
Ludmilla Cake
Modo the Dwarf, Gardener at UU
Morris Dancing
Mr. Arthur Winkins
Mr. Ixiolite the Banshee
Mrs. Drull, the Ghoul
Mrs. Evadne Cake
Mrs. Whitlow
Mustrum Ridcully, Archchancellor
Ned Simnel
Octarine Grass
Offler, the Crocodile-Headed God
Ole - "with milk"
Reg Shoe
Sargent Colon
Talking raven
Thaumometer
The Alchemists' Guild
The Amazing Maurice and his Educated Rodents
The Auditors of Reality
The Brass Bridge
The Bursar
The Counterweight Continent
The Dean
The Fools' Guild
The Guild of Merchants
The High Energy Magic Building and the Squash COurt
The Lecturer on Recent Runes
The Librarian
The Librarian
The Mended Drum
The Patrician, Havelock Vetinari
The Ramtops
The Rite of Ashk-Ente
The Senior Wrangler
The Shades
The Thieves' Guild
Things from the Dungeon Dimensions
Unseen University
Windle Poons
Yen Buddhism