Warning: the players of wizards will probably hate you forever for including this sort of thing in your campaign.
This creature — or magic item — or whatever it is — appears to be a large book like those used to record a magician's repertoire of spells. If scanned with appropriate spells or senses, it does radiate a magical aura, and it does have a life-force that could conceivably be detected if the means are to hand.
The arcanobibliovore feeds on stored magical energy, such as that in scrolls and spellbooks, and will attack and eat them if it can. It can "smell" the presence of such things within about ten metres.
Although it is capable of active hunting, moving about on hundreds of tiny centipede-like legs that fringe its "covers" (they lie flat when the creature is in "ambush" mode), its usual tactic is to allow itself to be "discovered" by a magic-user and to display a few enticing spells on its interior pages, in the hope of being stored with its prey. If left alone with a spellbook or scroll, it will tear it to pieces and eat any pages containing spells. It will target the richest food supplies first — that is, the highest-level available spells, leaving the low-level spells until last. The rate at which it eats is up to the individual GM, but I would suggest about 1-4 pages per minute.
The arcanobibliovore stores the spells it consumes as food reserves, and it gets fatter the more spells it consumes. It uses the energy slowly, using up about one spell-level a week. If it ever runs out of stored spells completely, it dies. It can be semi-tamed by supplying it with a reliable food supply.
The spells it consumes are not immediately destroyed, and the arcanobibliovore can make them available for study if it wishes. However it is not very intelligent, and cannot distinguish well between the individual spells held within its pages, so the spells it displays will be randomly chosen from all those in its reserves. On any given day, if it is in a good mood it can be induced to show 3-12 spells which can then be learned as if from any other spellbook (assuming they are of a level appropriate to the viewer).
Determine the arcanobibliovore's mood as follows: roll 1d6 and add +1 per spell-level of spells it has been fed by the supplicant within the last week; a score of 6+ indicates that it is in the mood to show its contents.
Note: Remember that the repertoire of spells available from the arcanobibliovore will change as it eats and/or digests its food. The GM can decide for him or herself how best to sort out what spells are still available on any given day.