The PCs don't have much time: they have to find certain mission critical bits of knowledge in a few ancient tomes on magic. It's not a matter of minutes, but they definitely can't read all the books from cover to cover.
What skill should the players roll to see if the PCs find the information? Spot? Search? Knowledge (Arcana)? A combination of these, or something we've completely overlooked so far?
(Were we playing Call of Cthulhu, I'd have them roll Library Use... :))
Clarification: It's not the books they have to locate. They have them, on their desk, and know that what they're looking for is in them. The better they roll (and the more detailed and imaginative ideas they come up with in describing their in-game research :)), the more they'll learn.
Example (not the actual case, that would be too long to describe): Imagine something like trying to find out, from a very detailed book on history, what consequences a King's laws had on the economy... four hundred years after he lived.