The Book of How

The librarian in a prominent Tellaran library, named Jimm Jeross, has been driven from his office, up on the second floor by demonic cratures who then move to take over the library themselves. They are tearing up books, tossing them about, destroying an incredible number of valuable volumes in a short matter of time.
Now these little demons have chased out the rest of the librarians and most (not all!) of the patrons. Those not accounted for (there are three) must still be in the building somewhere, perhaps hiding amongst the stacks, perhaps worse.
Jimm is frantic with worry about his books, and he wonders, wringing his hands, why these creatures have targeted their branch. He is less concerned about the people trapped, but of course there is that too. What can be done?
Heroes must enter the space and deal with whatever demonic presence is there. For a longer adventure there may be a catacomb beneath the building, ages old.
In the end, the little demon-beings are after one particular book, an ancient tome written in Abyssal, called The Book of How.. It gives instructions for sending the demons back (from the deep summoning rooms beneath the palace--a whole nother adventure there, spanning many levels) to their Abyssal origins. So the book has these instructions, but also lays out the entire history and locations of the major demon summons, their reasons, and their whys.
The Book of How ends up being the tip of a very large iceberg, as the heroes learn all about the demon beings in Tellarus, and how it may eventually affect the people and the very civilization of the great city.
So there are Quasits to be defeated, and lore to gain, and the opening up of an entirely new and much bigger problem. In other words, the heroes save the book, and the library, only to find out what the book says, and the history it reveals, and the vast problem it poses, the enormous task it suggests.