Arkham Mansion--Part 5

Rescue the Hostages

About now, the Joker will come on the PA and warn that you only have a minute-and-a-half before be blows a couple of hostages away, but it's an empty threat, so don't sweat the timer. Anyway, the event will drop you off in a small room, look for a second vent you can enter embedded in same the wall, but on the opposite half of the room as the one you just came from. It leads to a wall you can grapple up, into another vent, which will lead you to the top floor of the library.

Now that you're on the top floor of the library, toss a Batarang at the cable holding the chandelier to cause it to drop through the 2nd level floor, allowing you to follow it down to save the hostages by interacting with the present. Once they're done yapping, use Detective mode and inspect the nearby bookcase on the left to find Dr. Young's research notes.