SPOILERS BELOW For anyone who is interested in figuring it out for themselves:
To get inside the library, you can either:
- Throw a rock at the key sitting on the protruding plank on the roof to the right of the library's front door, hopefully causing it to drop, then using said key on the front door itself
Or...
- Climb up the scaffolding to the left of the library's front door (which requires you to push some boxes around to jump on, and then break a plank half way up the scaffolding by throwing an object or using a dagger, causing a piece of the scaffolding to fall and form a ramp),
- Enter the library either through falling down a level to the front balcony (taking some fall damage) and entering through the broken window, or landing on the 'side roof' on the right side of the library
- If you land on the side roof, you can either break the boards covering the hole with a crate sitting on it (by throwing an object or using the dagger) to enter the green key room, or, alternatively...
To get on the second floor:
- When entering the library from the front door, if you go left up the broken stairs and look straight ahead, you may notice a pile of books on the second floor landing. That pile of books holds the white key - either make a few jumps and try to grab it, or throw something at it until it falls.
- Now that you have the white key, you can go up to the side roof and unlock the door to the library's second floor.
Hope that explains it well enough. If anyone is interested, now that the jam is essentially over, I could post a video walkthrough of all possible solutions in the current build.