Ahh.. I forgot to mention one book:
Bushido Shoshinshu by Taira Shigesuke. An old book about the samurai ideals. There's two english translations of it; more recent by Thomas Cleary and older translation by A.L. Sadler (which is supposed to be a better translation).
Hagakure was already mentioned and I've also heard that book called Tengu Geijutsuron, but I have no idea if that's been translated or what it is actually like..