From Del Rey:
To resolve a clash over succession, the shogun Ieyasu Tokugawa has devised the ultimate contest. Two rival ninja clans, the Kouga and the Iga, will meet in a battle to the death. The victor will rule Japan for the next thousand years. But in the midst of this bloody war, an unlikely romance blooms between Gennosuke of the Kouga clan and Oboro of the Iga clan. Gennosuke and Oboro are the next leaders of their clans and their fates are inextricably bound with that of their families.
In the colossal fight, the star-crossed lovers are faced with a fatal choice between true love and destiny. Can romance conquer a four-hundred-year-old rivalry? Or is their love fated to end in death?

The first installment in the long-running Ninja Scrolls novel series.