Genealogy of Morals will expand upon some of the central themes of BGAE - especially the master/slave morality distinction. Birth of Tragedy is an early work which, whilst very interesting, is really Nietzsche's first attempt at expressing certain ideas, and is less clear than later works. Twilight of the Idols is good, and does a good job of giving various darlings of Western culture a good kicking! Other than that, Thus Spake Zarathustra is a poetic expression of his key ideas.

Interesting project you have. Yes, it would be interesting to see what Nietzsche thinks about feudal Japan. My feeling is that he would recognise the master morality in their social organisation, whilst being critical of the authoritarianism and the straight jacket of the social code. He is critical elsewhere of the emotional restraint of certain oriental cultures, and it seems that his own ideal was closer to the renaissance concept of the complete man - the warrior poet and philosopher artist.

Duty in Nietzsche is always a duty to oneself - or rather, to overcome oneself and upbringing, and strive to achieve one's highest personal ideal via intelligence, self-sacrifice, courage and creativity (at least, that's my interpretation!).