When the remake of It came out last summer I was glad to see a lot of people also noticing the creep-ass librarian standing over Ben's shoulder in the library scene - it was one of the moments in the film that got me, so much so that I reached over and shook my boyfriend's elbow to see if he'd noticed her (he had not).
There are enough little touches like that in the backgrounds of scenes to keep It working, I think, although I do feel as if people (like Xavier Dolan!) putting the film on their "Best of 2017" lists might be overestimating the movie a tad. But hey we like it enough not to truly begrudge it. Any movie that sets a scary scene in a library is a good movie.
The 1990 miniseries also featured such a scene, although it involved the adult characters (see here) - one of the few good scenes involving the adult characters in the miniseries that was any good, actually. But this scene in the remake doesn't remind me of that - this scene actually reminds me of the library scene in Dario Argento's film Inferno. They both involve the main characters being lured into the library's basement stacks where everything goes monochromatic and totally fucked up real quick...