Originally, I imagined the gun would be used for neutralizing the monster after some shots, to give you a few seconds to flee. On the other hand, I find there is a psychological impact on the player when they realize their gun is completely useless again this monster, forcing them to flee rather than to fight.
Or maybe I'd keep both these two options and let the player choose between a hard mode, where the gun would be useless against the monster, and the normal mode that would let you neutralize it.
Edit : I think it's funny that you find the page noises terrifying when it's just a sound I found in a free sound bank. I think it's mostly the fact that it takes place in the dark and the monster is off camera at first and seems to be getting closer for an eternity. That's kind of the point. I had to resynchronize the footsteps with Cthulhu's walk and I think I found the right rhythm. I'm pretty proud of it :)