Just wanted to jump in to clarify this one.
There's a very strong theme in the book that Pennywise, in his various guises, is not only an evil force himself but also infects the town and townsfolk. For example, horrible things happen through Derry's history but people, for whatever reason, are able to look the other way, are able to forget about the events, or in some cases join in on the violence. For example - a slaughter in a bar where the other patrons just keep drinking as if nothing is happening, and in another case, the town congregating on some visiting criminals and executing them in the centre of town.
I read this library scene, much in the same way as the old folks driving past the bullies and Ben right before they cut him up, as an example of this infection. The librarian, in that moment, is leering at Ben as he reads about the death of the kids at the factory. Maybe Pennywise is watching Ben through her eyes and enjoying his discomfort.