Lust geass, vol. 1 (Little, Brown Book Group)

Souta Takatsuki and his neighbor Rikka Amanome have been like brother and sister since they were kids, but his feelings for her run much deeper. With her birthday coming up, he wants to find the perfect gift, which presents itself in the form of a book from a mysterious shop he happens across.

The owner doesn't even charge him, saying he'll return to pay eventually...Souta is wholly unprepared for the insatiable passion the book inspires in Rikka, and it seems the ultimate price of this gift may be hefty indeed...