Dantalian no Shoka

Dantalian no Shoka summary:

The young man Huey took over the old mansion of his deceased bibliomanic grand father as well as the book collection that’s stored in there. But there was a condition: he also had to inherit the “Bibliotheca”.
When he inspects the mansion, Huey discovers a cellar filled with piled up books. In the midst of them he comes across a black-dressed girl who’s silently reading a book. She’s called Dalian and has a big lock hanging in front of her. She herself is the entrance to the “Bibliotheca of Dantalian” which contains the prohibited “Phantom Books”.