Nanto Iwareyou Tomo, Boku wa Tada no Kyuutei Shisho desu.

A boy, Serel, who works as a librarian at a certain royal magic library. He claims to be "just a librarian," but he chats with his childhood friend, Princess Fiona, manipulates advanced magic, and handles a huge workload all by himself; no matter how you look at it, he is not "just a librarian." Source: MU