"șoarece de bibliotecă"
It isn't an idiom we use in English - it's the literal translation of this Romanian idiom though, which is the equivalent of "bookworm". So it means the same thing in context (i.e. someone who loves to read and/or is kinda nerdy and spends a lot of time reading/in the library) but I meant that personally, I think "library mouse" sounds nicer than "bookworm". :) I also think nerds, geeks and bookworms/library mice rule. :)