Stuff DM's Need to Know
First of all, this is set in a Fuedal Japan-ish setting. Dur. But what makes it even better is that it uses the Tome rules, as compiled in the pdf download here. Now, while you can easily adapt all the rules, geography, and history to use the normal rules, keep in mind that the new classes and armor adhear to the Tome standards.
This is also sort of a revision of Oriental Adventures, except that instead of disguising it as a sourcebook when its really a campaign setting, this is just a campaign setting. It is recommended, however, that you still get Oriental Adventures if you want to play this campaign setting, since it has many Japanese weapons and magical items that cannot be reproduced here due to copyright.