As soon as you reach the
Training Facility, play until you've come across the part where you'll combine the
Lighter Fluid (located inside the kitchen) with Billy's own personal lighter.
From this point, head over to the "crank elevator" room (located upstairs from the main hall, enter the double door behind the statue, the door to the left past that, and then the first one to the right) and use the recently refueled lighter with the lantern that is being visually described by the near painting as the one to unlock the door by its side.
After entering the newly accessible room (It's a cozy but miserable one with a lot of bookshelves), slay the appearing zombies and climb up the ledge (Located at the end of the lane with some grenades flashing).
Now move slightly to the right to witness a mysterious bookshelf sticking out from the rest of its relatable setting, simply "nudge" it with Rebecca (Holding
).
Smile!
