If one is interested in Japan and its unique cultural history, Shogun is a good historical fiction to begin with. One will learn for instance why Japanese eat mostly raw fish and rice; why their houses were then mostly built with just wood and rice paper shoji screens; how the ceremonious chanoyu goes and what is the significance of it; what is the importance of believing in karma and not worry about things we have no control of, and much more.
Shogun also reminds its readers that while every culture has its better ways (e.g. Blackthorne's filthy culture then viewed baths as terribly dangerous to health) in certain aspects compared to others, culture in itself is unique and beneficial to its people as long as it serves them in their way of life. Evidently, as Mariko and Blackthorne found each other impossible to understand while at the same time completely irresistible, every culture on the face of earth does provide mankind something to learn from.