My personal favorite is this. At the top of a page of angular medieval text -- full of theological extrapolations and religious devotion -- is a cartoon of a deadpan dog.
"It's amazing to think that people doodled in medieval times in a similar way to how they doodle today," said Erik Kwakkel, a book historian at Leiden University, Holland.
"When you see the monks expressing their personalities, their sense of humor, it makes you feel like you're traveling back through time. It's like you're going through the keyhole and sitting right next to them."
Indeed, that dog would not be out of place in "The Simpsons."
'Medieval eye candy'
Kwakkel is making an unlikely name for himself on the internet by posting "medieval eye candy"
that he comes across during the course of his research.
And the doodles are by far the most popular.
"Normally, scribes would doodle or write snatches of lettering after cutting their nibs, to make sure they were the correct width," he said.
"These pen-tests ranged from the sort of scribbled lines that people still do today to words, names, full sentences or simple drawings. Sometimes we even find pretty good drawings."
These include funny faces with long beards, big hats or noses, as well as animals, unidentifiable creatures and even caricatures of teachers and colleagues.
In the majority of cases, the doodles were never intended to be seen. They were drawn on the outside of the first and last pages of a book, which were later glued to wooden covers.
But although the glue has obliterated a great many doodles and pen-tests, a variety has survived the test of time.
"They offer a rare glimpse into the informal or private world of medieval monks," Kwakkel said.
"Personally, I love the thumbprint, which was left by a careless scribe who spilled ink on his work. It seems so fresh and human, yet it happened 700 years ago."
For modern scholars, doodles and nib-tests can be more than mere curiosities.
When the scribes were trying out their quills, they often wrote in their own handwriting, rather than the heavily prescriptive styles demanded by their work.
This allows academics like Kwakkel to identify individual scribes -- who were rarely credited for their work -- and track their careers, including when they migrated across Europe.
Sometimes, they can also tell us about the classroom environment.
"I found one that is obviously a drawing of the face of a schoolmaster. You can recognize him from the teacher's hat on his head," Kwakkel said. "It was obviously drawn by a student in class, seven centuries ago."
Some may have had a deeper meaning at the time too, particularly the ones in the margins of the text (known technically as "marginalia").
One common trope was a drawing of a person's profile with a long, pointed nose. Kwakkel believes that this was intended to highlight a particularly important sentence, as indicated by the end of the nose.
Another example shows a snatch of musical notes together with the lyrics of a hymn. The monk was probably trying to remind himself of a song; this, Kwakkel says, is "one of the earliest example of musical notation that we have."
Treasure troves of secret manuscripts
But the aspect of his work that most excites Kwakkel is the uncovering of fragments of ancient texts, which he describes as "treasure troves" or "stowaways."
"After Gutenburg invented the printing press in 1436, the old handwritten manuscripts fell out of fashion. Some were thrown away, but others were chopped up and used in bindings and covers," he explained.
"I often take 15 students to a library for three days, and we carefully open up the bindings of medieval books to reveal what is inside.
"We have found many fascinating, rare manuscripts this way. It is magical, like uncovering a time capsule.
"I know there is a lot of stuff hidden out there in bindings waiting to be discovered, but I can't fathom what it might be."
His discoveries include the rough workings of medieval scholars who were translating texts from Arabic to Latin. Ordinarily, only the finished copies survive, so these manuscripts open up a new area of research into the translators' methodology.
But his favorite is a stash of 132 notes, letters and receipts that came from the house of a German nobleman.
One, dated 31 May 1486, was a note to a servant. "Could you please get me some wild roses, but make sure to include some that are still in bud," it says.
"That was breathtaking," said Kwakkel. "It was so casual and romantic. It as if it had been written just yesterday."