Played through the whole demo on Linux, didn't encounter any issues, though the mouse felt a bit too sensitive.
The atmosphere of the game is pretty good, but I feel the torch is too bright, and it could also use a flickering effect. On the other hand, I don't like the complete darkness in non-lit areas. You could add just a slight amount of global illumination to make basic navigation in the darkness possible, especially since the game has stealth elements where darkness is your best friend.
I liked the puzzles, though having to throw books on the shelves and hope they stick was a bit annoying. You could have a Thief-like system where you hold the object directly in front of you and can either throw or drop it exactly at the spot it currently is in. Otherwise everything was fun to figure out, throwing the green key through the small slit in the wall to avoid the glow from revealing me in the last level was a nice touch.
You definitely have to change the speed of the enemies. Right now getting spotted is pretty much an instant game over, though you can keep running as long as you don't get cornered since they're just as fast as you. Either slow them down to make escaping possible, or if you actually want getting caught to be an instant failure, make them fast enough to indicate that. I still managed to grab a colored key guarded by an enemy a couple of times and then run to the door to use it before dying to activate the checkpoint, and after reloading the enemy was reset back to its starting point.
From a gameplay point of view, I don't see a reason to have different keys for picking up inventory items vs. carriable items. If you add a key to gently place something down as mentioned before, it won't even be an issue near enemies if it makes no sound.
I noticed a bug with standing up from crouch, there's no checking for whether or not you have enough space to do that. In the last level, near the green key where there's a small tunnel made of crates you can use to douse the embers, you can stand up in the tunnel and clip into the crates.
Overall it's a really nice puzzle/stealth game though. Very interested in seeing where you're gonna bring it in the future.