The Mystic Archives of Dantalian

This story takes place in England just after world war 1. After the death of his Huey receives a mysterious key and according to his will, must take custody of the Bibliotheca Mystica de Dantalian in order to inherit his estate and all his possessions. Upon meeting Dalian, a child living at the estate, he learns that she is the guardian of the Archives which contain forbidden knowledge stored in thousands of magic books called "Phantom Books"

Just started watching this and I love it!