The victim was a 25-year-old male student in the School of Continuing Studies. His name has not been released.
There was no apparent trauma, and foul play is not suspected. Beyond that, authorities are saying little about the incident.
"I showed up around noon and spoke to the university police commander who said the library would be closed all day, and they wouldn't offer any further comment," said Nick Castele, editor, north by Northwestern.com.
An autopsy is scheduled for Monday.
University counselors will be available to any student who wants to speak with them.