Well, the Oriental Adventures book has a setting. It takes two chapters of world info and lotsa stuff sprinkled around the book, most notably the shugenja character class, the nezumi PC race, and a lot of clan-specific stuff. However, the book is not only about Rokugan. It is, basically, a 3E conversion of the original OA book, with Rokugan and some other stuff added in.
AEG is publishing more stuff on the setting, using the OA rules. There's a hardcover setting book with lotsa new stuff and a softcover monster book with lotsa new monsters, like the ones in OA weren't enough already.