Trigger warning: Discussion of 60s-era psychiatry practices and mental illnesses.

And what of the reveal at the end, that there’s an entire ward in the hospital of men who believe they are Vetinari, and that the staff maintains this fantasy to keep these people in line? That feels more like a joke than a realistic solution, and it came off more as Pratchett’s attempt to be clever than any sort of realistic commentary.

As strange as the Discworld is, the real world can somehow end up even stranger, and the Vetinari Ward has at least one uncomfortably Roundworld parallel. I'm pretty sure that Pratchett was consciously alluding to The Three Christs of Ypsilanti, a 1964 book about a psychiatrist's attempt to cure three different patients who each believed they were Jesus Christ. The psychiatrist, one Milton Rokeach, hoped to cure or positively affect their mental illness by having the three patients talk to each other and have group meetings where they each discussed their beliefs. Rockeach ultimately failed and along the way he did a lot of other stuff that even he eventually came to view as extremely unethical (making up false communications with people that he pretended existed via written letters with the patients, etc). The book is less fascinating than the premise suggests it will be, and may be extremely triggering, so overall I don't recommend it. They recently made the book into a dark comedy film which I have not seen (Three Christs, 2017) and which had poor reviews.

Apocryphally - Caligula's horse Incitatus was appointed a priest; possibly Caligula was planning to make it a consul or promised to do so or something else like that as well. Sources are contradictory and sketchy and primary sources super hated on Caligula (not without cause).

For those raised without classic 1960's Anglosphere children's books, "Where's My Cow" is an homage to P. D. Eastman's "Are You My Mother?". It's rather in the style of Seuss's work in terms of writing style, but with a less fantastic art / theme. It's a ~60 page color illustrated book about a baby bird; the mother leaves the nest while the egg is hatching to get food for him, and without supervision, the baby bird wanders around looking for his mother, and mistaking a number of other creatures for his mother - first a kitten, then hen, then dog, then cow. Then he begins mistaking a number of inanimate objects for his mother - car, boat, airplane. Finally he identifies an excavator (called a 'snort' in the book) as his mother; he enters the excavator's bucket, and the excavator returns him to the nest where he finally finds his mother.

Pieces of Eight. From the BBC:

It’s all down to Robert Louis Stevenson, and his classic adventure story, Treasure Island. The pirate Long John Silver’s parrot squawks ‘Pieces of Eight!’ repeatedly, and thanks to film and TV versions - which might well be more familiar than the book itself – it probably ranks among the world’s first great catch-phrases. In the last decade, Pirates of the Caribbean has come along to bump the great age of pirates and their treasure back up into the top layers of popular culture.

‘Pieces of Eight’ is one of many names for the large silver coins of the king of Spain, a multiple of the basic Spanish denomination, the silver real: so a piece of 8-reales, peso de ocho reales, or peso. It is also the original silver dollar, a name that starts out as a place in the Czech Republic in the sixteenth century and ends as the currency of the modern USA.

Pieces of eight pretty much ruled the monetary world from the 1570s till the French Revolution, the main vehicle for the transfer across the globe of silver from the great mines in Mexico and Bolivia in Spain’s American empire. It was a coin that would be familiar in every part of the inhabited world.

Throughout this period only infinitesimal quantities of pieces of eight ever fell into pirate hands. The Spanish operated a highly successful system of security and transport, the threats to which came from organised enemy fleets, not rag-tag pirate crews. Most of their silver sailed around the world paying for wars, encouraging trade, changing the fates of empires. You cannot write a history – especially an economic history - of the early modern world without engaging with pieces of eight. Pirates of the Caribbean merely nibbled on the insignificant fringes of this world.

Cardigan lead the Charge of the Light Brigade, subject of a really famous and well-known poem by Tennyson. Link to the poem online. It's a paean to the glory of charging straight into the enemy, pro-military poetry / propaganda.

Oh cool! I hadn't heard that before. I really ought to read those through sometime.

The amount of time to wait for the marriage is clearly a reference to the description of eternity from Joyce's A Portrait of the Artist as a Young Man. To quote from the Project Gutenberg e-book:

You have often seen the sand on the seashore. How fine are its tiny grains! And how many of those tiny little grains go to make up the small handful which a child grasps in its play. Now imagine a mountain of that sand, a million miles high, reaching from the earth to the farthest heavens, and a million miles broad, extending to remotest space, and a million miles in thickness; and imagine such an enormous mass of countless particles of sand multiplied as often as there are leaves in the forest, drops of water in the mighty ocean, feathers on birds, scales on fish, hairs on animals, atoms in the vast expanse of the air: and imagine that at the end of every million years a little bird came to that mountain and carried away in its beak a tiny grain of that sand. How many millions upon millions of centuries would pass before that bird had carried away even a square foot of that mountain, how many eons upon eons of ages before it had carried away all? Yet at the end of that immense stretch of time not even one instant of eternity could be said to have ended. At the end of all those billions and trillions of years eternity would have scarcely begun. And if that mountain rose again after it had been all carried away, and if the bird came again and carried it all away again grain by grain, and if it so rose and sank as many times as there are stars in the sky, atoms in the air, drops of water in the sea, leaves on the trees, feathers upon birds, scales upon fish, hairs upon animals, at the end of all those innumerable risings and sinkings of that immeasurably vast mountain not one single instant of eternity could be said to have ended; even then, at the end of such a period, after that eon of time the mere thought of which makes our very brain reel dizzily, eternity would scarcely have begun.

I'm pretty sure this comes up in Good Omens as well.

The Angel of the Iron Book also echoes two different things...

The first is The Angel With The Little Book, from Revelation 10:

And I saw another mighty angel come down from heaven, clothed with a cloud: and a rainbow was upon his head, and his face was as it were the sun, and his feet as pillars of fire:

And he had in his hand a little book open: and he set his right foot upon the sea, and his left foot on the earth.

Note: various people have tried to make art of an angel as described here and tried to paint it as a serious and not intrinsically hilarious getup; judge for yourself how well it worked...
.

The other thing the Angel reminds me of is a minor character in a book series that Mark hasn't read yet; Author: Qbhtynf Nqnzf

Va Qbhtynf Nqnzf' Uvgpuuvxre'f Thvqr Gb Gur Tnynkl, Obbx Gjb: Gur Erfgnhenag Ng Gur Raq Bs Gur Havirefr, gurer vf n Terng Cebcurg Mnedhba jub vf sberfunqbjrq gb pbzr ng gur raq bs gur havirefr naq gura fubjf hc naq vf irel njxjneq naq hapbzsbegnoyr zhpu va gur znaare bs guvf Natry Jvgu Gur Veba Obbx. Pbairetrag ribyhgvba va pbzrql, be pbafpvbhf be hapbafpvbhf vafcvengvba?

Suggestion for #1: (spoilers for Science of the Discworld #1):

Bar bs gur qrsvavgr gurzrf gb gur fpvrapr frpgvbaf bs Fpvrapr bs gur Qvfpjbeyq #1 vf ribyhgvba. Pbafvqre fbzrguvat yvxr bar bs gur ribyhgvba-bs-uhznavgl vzntrf jurer vg fubjf n yrsg gb evtug vzntr bs bhe erprag ribyhgvbanel uvfgbel zbaxrlyvxr perngherf, ohg nqq va Gur Yvoenevna sebz gur Qvfpjbeyq ng gur raq, nf vg vf n Jvmneqf-pragrerq obbx. Abgr gur vzntr yvaxrq sebz jvxv vf bar bs n ynetr ahzore bs vzntrf bs guvf glcr, zbfg bs juvpu ner sne zber qrgnvyrq naq nccrnyvat - ohg V qba'g xabj jung bs gurz ner va gur choyvp qbznva...

There was previously some discussion of printing and movable type in Jingo, where it was a joke that was not centrally relevant to the plot. From Jingo:

The wizards of Ankh-Morpork had been very firm on the subject of printing. It’s not happening here, they said. Supposing, they said, someone printed a book on magic and then broke up the type again and used it for a book on, say, cookery? The metal would remember. Spells aren’t just words. They have extra dimensions of existence. We’d be up to here in talking soufflés. Besides, someone might print thousands of the damn things, many of which could well be read by unsuitable people.

The Engravers’ Guild was also against printing. There was something pure, they said, about an engraved page of text. It was there, whole, unsullied. Their members could do very fine work at very reasonable rates. Allowing unskilled people to bash lumps of type together showed a disrespect for words and no good would come of it.
The only attempt ever to set up a printing press in Ankh-Morpork had ended in a mysterious fire and the death by suicide of the luckless printer. Everyone knew it was suicide because he’d left a note. The fact that this had been engraved on the head of a pin was considered an irrelevant detail.

And the Patrician was against printing because if people knew too much it would only bother them.