Shoujo wa Shoka no Umi de Nemuru

Phil is a young man who is in training to run a bookshop. He loves books, and his mentor sends him to a nearby monastery that is said to be in trouble with money, to offer to buy books out of their library. Phil has heard rumors of their library, and is eager to see it for himself in any case. The kid who answers the door, however, refuses to let him enter. After he finally makes it inside the monastery, he finds that tragedy seems to have struck the fellowship and that "kid" is actually a beautiful girl who seems to be a noble. Things are more complicated than they seem, and it looks like Phil is going to be in over his head!

(Source: MangaHelpers)