During the fight, the Librarian will call for the Guard (the Golem and the Professors). If you cleared them, you don't have to worry.
You need to look at her casting bar. When she's casting "Project Knowledge" you need to crowd around her: the farther your are, the more the AOE hits you for. When she's casting "Gathering Knowledge" you need to step back in order to avoid her AOE. The casting time for those 2 abilities is long, you have time to gather around her or to step back.
Her spell Call to Knowledge will cause book ads will appear. Burn them as they do much damage.
She has a nasty personal buff that can be dispelled; she may recast this during the fight so be aware.