Carusmetus’s nervous finger groped through the dust like polyps. They alighted on the spine of a book, "How to Survyve, Volume I: Cavernes and Undergrounde Playces". That’d do. It dropped into his sack and he dashed off between the bookstacks.
His mind raced as he weaved his way down and down; had he all the books he needed? Speleology, safe-cracking, first aid…they were all there in his sack. He hurtled along the empty corridors of the Great Library and down its voluminous stairwells. At this time of night there was no-one to mind his hurry.
The black door of the Dark Magik department eyed him warily from the far side of the reading room. Breathlessly he approached and whispered his business to it. The door winked and eased open and he flung himself into the rooms beyond, lit by black candlelight. What was the classmark he was seeking? "DM04.b.567.900"
The other man watched Carusmetus from behind the shelves, and only pursued when the wizard had gone far enough into the shadow that he wouldn’t see he was being followed.
Carusmetus stood by the shelf. He lifted down the leather-bound magicbook and wiped the dust off to reveal the classmark. "DM04.b.567.900" Perfect. He stopped breathing. Was that a cough he’d heard? A dry and horrible cough, as if someone unused to the thick and dusty atmosphere of the Library was standing nearby. Or was it just some sleepless books? He only managed a tiny yelp before dying. He dropped his sack and his head lolled bleeding horribly on the floor.
Meanwhile, inside the sack sit an unusual combination of books, all listening to the unpleasant incidents outside. Upon the wizard’s death, all hell breaks loose - the books of Dark Magic on the nearby shelves awake and begin their disturbed yowling and caterwauling.
The player characters are the books in the sack, with titles instead of names and abilities according to their contents. For example, "How to Survyve, Volume I: Cavernes and Undergrounde Playces" (or "Volume I" to his friends) is a fairly stereotypical dwarf-like character (small, sturdy, leather-bound and quite dog-eared). A manual on first aid might be a clerical-style character; a book on safe-cracking might be a thief; a newspaper might be a bard; a textbook on martial arts would clearly be a sleek paperback ninja.
The characters must escape unharmed from this dangerous area of the library (with many worrying books like "The Curse of Lycanthropy" or the terrifyingly non-specific "Grotten’s Bestiary" which can take on the abilities and manner of any of the beasts listed within), and then work out WHY the wizard was murdered and WHY he collected such an esoteric set of books in such a hurry.
I like this idea, because the PCs themselves are clues in the mystery…
Possible solution to the mystery
The Tyrant King, sick of opposition from the city’s Scholars to his warlike foreign policy has decided to punish them by destroying their dear library. He has discovered of the secret caverns beneath the Great Library and intends to put a large pile of explosives there, to demolish the building in a "freak magical accident". Perhaps Carusmetus was a traitor who revealed the existence of the caverns to the King, who then repented and decided to try and prevent the plot himself (hence the strange selection of books - to help him in his quest). The King, anticipating this, sent an assassin to deal with him.
So, roleplaying a book should be fun (though perhaps only in a bizarre one-off session) - books have personalities and they could really be exaggerated. A pompous author would lead to a pompous character, maybe? But how do books attack? How do they move?
* Clearly they move by opening and closing to crawl along with their spines upturned. Books such as "Birds of Northern Myoleria" or "Flight for Beginners" will obviously be able to fly (by flapping).
* Armour-class should correspond to the quality of binding (e.g. leather binding would be like plate-mail, but paperbacks would be vulnerable).
* Attacks would be by snapping, buffeting, giving paper-cuts, maybe diving and swooping. I recommend putting together some complex eight-way rock-paper-scissors system to resolve combat (as it’s a one-off session you can afford to experiment!). So each book would have a limited number of ways of attacking (e.g. snapping and buffeting; or diving and swooping) and would also be able to dodge. For example:
DODGE beats BLUDGEON,SNAP,SWOOP and avoids damage
BUFFET beats DODGE,DIVE,SNAP for 3 points of damage
BLUDGEON beats BUFFET,CRUSH,CUT for 4 points of damage
SNAP beats BLUDGEON,CUT,DIVE for 3 points of damage
CRUSH beats DODGE,BUFFET,SNAP,CUT for 6 points of damage
CUT beats DODGE,BUFFET,SWOOP for 4 points of damage
SWOOP beats BUFFET,BLUDGEON,SNAP,CRUSH for 3 points of damage
DIVE beats DODGE,BLUDGEON,CRUSH,CUT,SWOOP for 4 points of damage
(WARNING! This is just an entirely made up and untested example; it’s best that you tweak it - it’s just to give you an idea of what I mean.)
Inventing hand gestures for these attacks could also be fun.
* Hit points could correspond to number of pages. Iain suggested that abilities could be lost with pages (for example, if you lost your page on "how to climb really steep walls" and had to climb a really steep wall, you probably couldn’t do it. I figured a sensible way of keeping track would be to have a number of pages for each skill equal to your percentage chance of success in that skill. Then, as you lost pages (hit points) you’d actually randomly lose skill percentiles and when you reached a certain level of dog-earedness you’d fall apart, ready to be sent to the bindery.