As The Ink Dries - ViciousVenin (Ao3)

Description:

Frank has a hard time not hating his job. He's so close to his dream of working as a reference librarian and yet, due to a few stupid choices, he's stuck doing processing and mending. Tasked with the same mind-numbing chores day after day, Frank feels he'll die of boredom -- that is, until Gerard is hired.

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This one is a slow burn and it really makes you feel everything the characters do. I'm a sucker for a good library AU.