Inspiration for actual Games Workshop Asian inspired materials were hinted in the mid to late 1980s, but never delivered on. Nippon and the east in general is mentioned it in the 3rd Edition Rule book, had an Ally section in Warhammer Armies, but then it ended with the advent of 4th Edition Warhammer and teh changes it entailed. It is important to note that 2nd Edition Warhammer Fantasy roleplay did have a Nippon list that with a few tweaks is quite usable.
Fortunately the world of the internet allows for this constraint to be eliminated.
In Warhammer Nippon (Nihon) is analogous to Japan of our world, duh right? What it is NOT? Its not... mythical China. This can't be overstated because all too often in Warhammer circles I hear suggestions about a Nippon army that would make it more like Cathay... If I wanted to create a fantasy Cathay army out there there is a number of resources, but not the point of this blog post.
So in no particular order here are two of the best inspirations for a Nippon themed world and Nippon army that can help, in my opinion of course.
Shogun- The book written by James Clavell and the TV series are both excellent. It is a pseudo historical in the telling of the late 1500s and the unification of Japan by Tokugawa Ieyasu the first Tokogawa Shogun in the form of "Toranaga".
Shogun is the quintessential TV Mini series of the 1980s and in six parts. Richard Chamberlain starts as Pilot-Major Blackthorne. Blackthrone is based on the real life exploits of an English sailor who was shipwrecked in Japan in 1600, William Adams.
Shogun covers nearly all aspects of late 1500s Japanese life from teh peasants to ninja to the start of the geisha, betrayal, honor, bravery as well as romance. The ambush of bandits at night in a village, ships, ninja! The only thing it lacks is the actual Battle of Sekigahara which it all leads up to.
The rising tension of the plot makes an excellent backdrop for Nippon vs Nippon battles or perhaps more likely as I do: reasons for mercenary Nippon to go and plunder the Warhammer World.
Akira Kurosawa. Ran is essentially King Lear, but far richer in the telling. Ran is widely regarded as one of the greatest movies of all time, I agree. Any Top 10 movie list in the All Time category that doesn't include it is no Top 10 at all.
Ran is rich in its visuals and massive in scale. I think perhaps the only true way to do it justice is to see it in a movie theater (which I never have, but would love too).
A side note is that Ran has one of the best soundtracks for any Asian inspired game. I listen to it often when working on one of my Nippon army projects.
Ran is indispensable to a Warhammer gamer interested in a Nippon army as it has literally all the right army units on display throughout (remember I use the 2nd Edition Nippon list for 3rd Edition Nippon armies).
Other good sources include but are not limited to:
Oriental Adventures by Zeb Cook (1st Edition AD&D)- a lot of grogs have a hangup about this work, tell them to pound sand... A lot of people seem to want it to be "epic China" which its not... I think the rules are quite well done, especially considering the time frame.
Kobu of the Two Strings. This might seem surprising but I found this extremely well done.
The Seven Samurai- many people don't realize that they HAVE seen this movie in the form of the movie the Magnificent Seven. The soundtrack of The Seven Samurai is great too. Oh and it also inspired George Lucas with the droids in the original Star Wars trilogy too.
Japan- Memoirs of a Secret Empire- a documentary narrated by Richard Chamberlain, so short ( I wish this was 40 hours long!) but it is excellent. Its not really a secret but meh its just the title.
As a postscript: this should not be taken that I dislike a Cathay army or its inspirational sources, far from it. I think epic China is a fine topic and if I ever did an army inspired by it I'd go with something like Kung-Fu Panda! Panda bear warriors would be very cool ;) Clay Warrior golems, river dragons, etc