Riot Vol 1

From the VIZ Media edition

Billy the Kid, a young outlaw, attempts to steal Riot, a sacred text that contains untold treasures of an ancient kingdom, not to mention half the secrets of the universe. But his attempt is foiled, and as Billy the Kid lies dying, mortally wounded by Axel, the beautiful woman who guards Riot, the book itself strikes a bargain: Billy can have insight into the volume's secrets as long as he protects Axel for all his days.

Source: Anime News Network Encyclopedia