Uncanny Japan covers some of the legends from Japan which could be adapted for L5R https://www.uncannyjapan.com/
The Samurai Archives Japanese History Podcast. http://samuraipodcast.com/ This is a treasure trove of information, along with the Samurai Archives wiki , for varied subject son how samurai may have operated, details on economics (and why most of the editions didn't really bother with much detail), how samurai politics worked, and a bunch of other information that may be of use to GM's
A Short History of Japan . While a little digging is required, some historical context for the Warring States era may give players and GM's ideas for adventure in Legend of the Five rings http://ashorthistoryofjapan.libsyn.com/