As I've mentioned before, in my day job I'm a professor of Asian history, specifically military history ca. 1550-1800. My main focus is China, but I teach pretty much everything. So over the years I've read lots of stuff and a few Japanese-related works stick out as being of potential interest to L5R players. I have used all these books in my classes over the years.

The Book of Yokai by Michael Dylan Foster: https://www.amazon.com/Book-Yokai-Mysterious-Creatures-Japanese/dp/0520271025/ref=sr_1_1?dchild=1&keywords=book+of+yokai&qid=1590889833&s=books&sr=1-1

Japandemonium Illustrated by Toriyama Seiken: https://www.amazon.com/Japandemonium-Illustrated-Encyclopedias-Toriyama-Sekien/dp/0486800350/ref=sr_1_1?crid=1BXZL6IH5VJ5L&dchild=1&keywords=japandemonium+illustrated+the+yokai+encyclopedias+of+toriyama+sekien&qid=1590890127&s=books&sprefix=Japande%2Cstripbooks%2C200&sr=1-1

The First Samurai by Karl Friday: https://www.amazon.com/First-Samurai-Legend-Warrior-Masakado/dp/047176082X/ref=sr_1_1?dchild=1&keywords=karl+friday+the+first+samurai&qid=1590890193&s=books&sr=1-1

Samurai, Warfare, and the State by Karl Friday: https://www.amazon.com/Samurai-Warfare-State-Medieval-History/dp/0415329639/ref=sr_1_1?dchild=1&keywords=karl+friday+samurai%2C+warfare%2C+and+the+state&qid=1590890246&s=books&sr=1-1

The Pocket Samurai by William Scott Wilson, trans., [Includes Book of Five Rings]: https://www.amazon.com/Pocket-Samurai-Shambhala-Classics/dp/1611802164/ref=sr_1_1?dchild=1&keywords=The+Pocket+Samurai&qid=1590890357&s=books&sr=1-1

The Diary of Lady Murasaki by Murasaki Shikibu [Author of Tale of Genji but this is much shorter & still gives great ideas for courtly intrigue and behavior]: https://www.amazon.com/Diary-Lady-Murasaki-Penguin-Classics/dp/014043576X/ref=sr_1_1?crid=2A4XQ983MAL47&dchild=1&keywords=diary+of+lady+murasaki&qid=1590890432&s=books&sprefix=Diary+of+Lady+Mura%2Cstripbooks%2C275&sr=1-1

The Pillow Book by Sei Shonagon: https://www.amazon.com/Pillow-Book-Penguin-Classics/dp/0140448063/ref=sr_1_1?crid=38OXO9MBP7S1T&dchild=1&keywords=pillow+book+of+sei+shonagon&qid=1590890539&s=books&sprefix=pillow+book+%2Cstripbooks%2C377&sr=1-1

And finally, my own book, which is about Hideyoshi's invasion of Korea from 1592-1598: A Dragon's Head & A Serpent's Tail: Ming China and the First Great East Asian War, 1592-1598 by Kenneth Swope: https://www.amazon.com/Dragons-Head-Serpents-Tail-Commanders-ebook/dp/B007QTZPM6/ref=sr_1_1?dchild=1&keywords=a+dragon's+head+and+a+serpent's+tail&qid=1590890676&s=books&sr=1-1

Much appreciated on sharing this. Do you have the links to the materials you use for Chinese, Korean. Malay, Thai, Mongolian, Cambodian, and other cultures? Separated by ethnic group, I still think there is value to reading them when preparing for an L5R game.

Well, for Chinese stuff, that would be a massive list since that's my specialty. I could literally recommend books on every historical period and era so if you have specific times you're interested in, let me know. My specific academic focus is Ming-Qing China (1368-1911), especially military history and the history of peasant rebellions and secret societies. Most of my work has been on the Ming response to Hideyoshi's invasion of Korea; the fall of the Ming, and the Ming-Qing wars. I'm currently writing a biography of General Zuo Zongtang (1812-1885), of "General's Chicken" fame here in the USA.

For Thailand, the best single volume survey is David K. Wyatt's, Thailand: A History. Interestingly, his son, James, wrote the D&D 3.0 Rokugan book and a lot of other Oriental Adventures & other gaming materials for D&D. Morris Rossabi's book, Khubilai Khan, covers his reign, including the Mongol invasions of Japan. Timothy May has published accessible stuff on Mongol military history and culture. Anthony Reid has a great two volume set that covers all of Southeast Asia, Southeast Asia in the Age of Commerce, but it's very academic. David Chandler's History of Cambodia is pretty good. For Japanese pirates, I'd recommend Peter Shapinsky's Lords of the Sea. On the Japanese pirates ( wako ) in East Asia, there's Kwan-wai So's Japanese Pirates in Ming China, which is old, but a bit more readable than some other works. Robert Antony's Like Froth Floating on the Sea is pretty good for pirates in China, 1550-1800.

That should get you started...

Also, there are several good Korean primary sources in translation that might be of particular interest to L5R players.

These are The Memoirs of Lady Hyegyong: https://www.amazon.com/Memoirs-Lady-Hyegyong-Autobiographical-Eighteenth-Century/dp/0520280482/ref=sr_1_1?crid=3FNY57HKFOKSP&dchild=1&keywords=memoirs+of+lady+hyegyeong&qid=1590972862&s=books&sprefix=memoirs+of+lady+hye%2Caps%2C313&sr=1-1

The Book of Corrections: https://www.amazon.com/Book-Corrections-Reflections-1592-1598-Monograph/dp/1557290768/ref=sr_1_1?dchild=1&keywords=the+book+of+corrections+by+yu+songnyong&qid=1590972948&s=books&sr=1-1-catcorr

A Korean War Captive in Japan: https://www.amazon.com/Korean-War-Captive-Japan-1597-1600/dp/0231163703/ref=sr_1_1?dchild=1&keywords=korean+war+captive+in+japan&qid=1590973005&s=books&sr=1-1

The second two are connected to Hideyoshi's invasions, but seem to have recently gone out of print or gone up in price. I just used them last semester and they were cheap then, so maybe they'll go down in price again.

19 hours ago, Masakiyo said:

Most of my work has been on the Ming response to Hideyoshi's invasion of Korea;

That actually sounds interesting!

I don't know enough about japan before 1447 or so

Actually the invasion was late Sengoku era, 1592-98. So firearms played a huge role. Not sure how that would work in Rokugan, but one thing I'd thought of is giving different magical abilities to the different sides that could emulate the different applications of firepower if someone wanted to base a campaign on this. The Unicorn use magic differently because of foreign contact, right?

21 hours ago, Masakiyo said:

The Unicorn use magic differently because of foreign contact, right?

Correct. Meishodo is something based more on classic Arabian Nights style 'bind a djinn' than Rokugan's normal 'pray and ask nicely' approach

Curiously, Meishodo looks a lot like Pokémon, with the difference that you might not get the kami back after being released.