I just typed it from the audio, which isn't always reliable. I had every word but "topo," and just couldn't believe that was it. However, I couldn't think of any word that sounded similar, so I put "topo" and was surprised to find it was correct. So, I've heard of "church mice," but never "library mice."
I'm not a native speaker, but I note that it does show up in Italian dictionaries. "Lettore accanito, assiduo frequentatore di biblioteche, erudito che passa il suo tempo in mezzo ai libri a studiare e divorare volumi, come un topo chiuso in una biblioteca a rosicchiare pagine." (An avid reader, a frequent visitor to libraries, a scholar who spends his time in the middle of books studying and devouring volumes, like a mouse locked in a library gnawing pages.)