Mortimer's Mistake

The wagon barrelled along the street, angry mob behind, changing lanes and riding on sidewalks, anything to get ahead.
"Mums! You know where the library is, right?" yelled Carl.
"Lllllllllllllllleeeeeeeeeeeeeeeeefffffffffffffftttttttttttttt heeeeeeeeeeeeerrrrrrrrrrreeeeeeeeeee? Ooooooooooooooorrrrrrrrrrrr Rrrrrrrrrrrrrrrriiiiiiiiiiiiiiiiiiiiiiiggggggggggghhhhhhhhttttttttt?" Mums intoned over the course of five minutes.
"Way back there?" asked Mortimer. "Should have been a left."
"DDDDDDDDDDDDAAAAAAAAAAARRRRRRRRRNNNNNNNNNN."
"Maybe you'd better let me drive?" asked Carl.
Mums obediently, if slowly, moved over.
"Alright, a U-turn takes us into the heart of the mob, so we're gonna have to go around the block. And you're gonna have to jump out, cause we can't afford to stop."
"Got it."
"Fair ze well, Mr. Mortimer," said the Count. "Until ve meet again."
And with that, Mortimer jumped out of the wagon and into the street, ducking in the library door just as the mob overtook him.
He took a minute to take in his new surroundings. The library was as dead as the last time he'd visited, and the old librarian was snoring at the desk. This time there was only one book. A History of the Third Age had not been reshelved, and The (In)complete Book of Ferret Recipes was gone. No, not gone, he realized as he glanced around the room. It was now being used under the leg of the single ratty armchair, making it only slightly less unstable.
In their place was a new book (new in the sense of not being there on his last visit; there was nothing new about it) entitled How to Raise and Train an Airborne Ungulate: An Owner's Guide. There was something off about it.
"Uh, hi." said Mortimer.
The librarian did not awaken.
"Ahem hem."
Nothing.
"EXCUSE ME SIR!"
"Huh? Whu?" the man mumbled.
"I need to see that book again, A History of the Third Age. It's very important."
SLowly, the man reached for his spectacles, balanced them on his nose, and began thumbing through a huge ledger. Mortimer wondered what could fill a book like that in a library with a possible total of three books. Of course, it was possible that the library was merely so immensely popular that all the books were checked out and any given time.
"I'm sorry, sir. That title is checked out."
"What? I returned it only yesterday."
"That's right sir, and some men checked it out almost right after you left. They also donated a new book to the library. They said if you came back I should recommend it to you."
Mortimer turned around again and reached for How to Raise and Train an Airborne Ungulate: An Owner's Guide, with a sickened feeling in his chest. It couldn't be.
The book was merely a binding with a single piece of parchment inside. It said, in words cut and pasted from another book, "If you ever want to see THIS BOOK that you checked out or YOUR BELOVED PET (STeed) again, you'l come to [here a picture was inserted, a crudely drawn upside-down duck] at 10 on the 10th."
Mortimer grabbed the note and sped out of the library, running on Eagle's Wings all the way to the Inverted Duck, where he'd left Narrin. Sure enough, she was gone.