Ron had, more or less, gotten used to the idea that if you wanted to find Hermione, if she wasn't right there sticking her nose in it, then she was in the library. It had been bloody annoying in his first few years of school but now it had somehow shifted from annoying to endearing. That is, when he wasn't trying to actually locate her and had to go tramping through stacks and stacks of books to find her, with Pince hushing him and glaring at him, like it was his fault that his feet actually made sound when he walked.
He found her finally, and sat down across from her with a frown and grimace for the moldy old book and then her as well. "You've dust on your nose," he said conversationally.