Reluctantly, I can't upvote this. It has some excellent details — the mouthless creature, the latter part of the argument between Marshall & Carter, the detail about Crowley & the Library (awesome), and the entire closing scene is all-around excellent.
However, the bulk of the story is spent with Carter freaking out at excessive length, then Marshall & Carter both freaking out at excessive length, with zero payoff. It's fine to have the freaking out, but much of it could (and should) be cut. And regardless of that, there's just not enough actual content here. You don't necessarily need that much more, but the scenes are too straightforward; you could get waaaay more out of them, especially the first two (the final scene is still straightforward but works for me anyway).
Yes, I know, you badgered me to read this several times when it was still in draft stage, so this feedback could've been more helpful then. Sorry! D:
EDIT: I ended up upvoting anyway on re-read. Largely due to enjoying the details I mentioned above.