Decapitation: Kubikiri Cycle - The Blue Savant and the Nonsense User

Nineteen-year-old Ii-chan accompanies his childhood friend, the blue-haired computer genius Tomo Kunagisa, to Wet Crow's Feather Island. There, the exiled heiress Iria Akagami entertains a rotating cast of brilliant guests: an artist, a cook, a fortune-teller and a scholar. When one of them is found beheaded inside a locked room, the self-described ordinary boy in a house full of geniuses has to work out who among them is lying. (Source: NISIOISIN Wiki)

Decapitation Cycle: The Blue Savant and the Nonsense User (Original: Kubikiri Saikuru: Aoiro Savan to Zaregoto Tsukai) is the first novel of the Zaregoto Series, written by Nisio Isin and illustrated by Take. It was originally released on February 5, 2002 in Japanese.