Demons are trying to overrun the world and erase all of the Words of Creation so they can rewrite reality as they want it. You can banish demons for a year and a day if you know their true name. The library has a history book that mentions a wizard who knew many true names, but it doesn't actually list the names. The party doesn't know this.
The party is just exploring a landscape that used to be a mighty nation before a cataclysm destroyed it. Now they're helping to establish a colony and loot the ruins. They find an old haunted library and helps a new librarian get control of it. She discovers a spell that lets you enter books and look around, but everything inside is static, so I guess that's no big deal. It's really just like, "Look, I made a lifeless demiplane that happens to look like what's inside that book. You can't take anything out, though, so it's only really useful if you hate reading."
In a couple weeks, I'm going to run the session where a mighty demon comes tromping through the party's turf, and its minions are looking for a particular book. The First Book, written by God Himself (well, technically by one of his angels, but close enough). It contains the initial words of creation, and the librarian realizes that if the PCs can get that book, they can splice in the history book from before and make the text within come alive (i.e., remove the cover and binding, stitch in the pages from the history book you need, then rebind it). Then they can enter the book, have an adventure to find and talk to the wizard, learn the demon's true name, and banish it.
This is the first of a handful of quests in what I've named my Tome Raider campaign.
