I'm currently reading 'autumn bridge' by takashi matsuoka, the sequel to his excellent 'cloud of sparrows'. They are primarily books about Japan in the 19th Century, but autumn bridge actually jumps around from the 14th to the late 19th century. This may sound confusing, and it is (especially when your bookmark falls out), but I'm really enjoying it.
Through reading these two books I've learned lots of new Japanese words and lots of Japanese history. The trouble is that I've no idea what is and isn't history. Like the Bernard Cornwell books they contain information on a number of major events and characters and are very well researched. Unfortunately whilst it is generally easy to determine who is and isn't 'made up' in the Cornwell books my lack of knowledge of Japan makes this much more difficult.
The primary rivalries among the samurai in the 19th century result from the outcome of the battle of Sekigahara in 1600. This is commonly known as the Realm Divide and cleared the path to the shogunate for the Tokugawa clan, the last shogunate to control Japan.
James Clavell's Shogun is also a fictionalized account of the rise to fame of the first Tokugawa shogun (thinly disguised as "Toranaga"), culminating in the Battle of Sekigahara.
These days one of Sekigahara's few non-martial attractions is the profusion of fireflies in the area. Lake Mishima is home to a particularly famous firefly spot housing hundreds if not thousands on a good day. A Firefly Festival is held there in the early summer.