A small creature that has evolved in the highly magically charged environment of the Unseen University. Because magical books are often dangerous, the .303 bookworm has evolved to eat books at incredibly high speeds. It can often be found shooting out of books, ricocheting off the opposite wall. This presumes that unlike other maggots, it has an incredibly hard carapace, as a normal pupated lifeform at those speeds would just go "splat", which means it misses out on the next stage of its personal development (the chrysalis followed by the mature insect). In fact, going "splat" at the worm stage would mean the species dies out, as only the mature adult may breed the next generation of .303 bookworms... so there must be some sort of hard resilient carapace enabling the creature to survive the ricochet. By extension, a direct hit on anyone standing in the way would be fatal?). This makes it the fastest animal on the Disc (saving only the Ambiguous Puzuma– unfortunately, a race would be hard to arrange and impossible to judge, due to quantum uncertainty.)
.303 was for over sixty years the standard rifle calibre of the British armed forces. It is clear that librarianship on the Disc often requires a bullet-proofed vest or an armoured book trolley whilst re-shelving.
We can also make an informed stab at the size of a .303 Bookworm: if its base calibre, or width, is a uniform .303", by association its length must be about 1 and 5/8" (ie, the length of a standard .303 bullet) and for aerodynamic efficiency, there would be a gradual taper along its length to a blunt point. As it takes in ink, gilding, binding glue and colouring matter in addition to edible parchment, one might hypothesise that the creature does not excrete as such: any indigestible matter, perhaps metals extracted from book-related matter it consumes, are extruded out to form the aforementioned hard carapace, which takes the form of a tapering metal cone... (Would the genesis for the Gonne have been Leonard of Quirm's harmless and inoffensive nature studies of the life-cycle of the .303 Bookworm?)