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A good book that is a little general but one that I found useful is The Samurai sourcebook by Stephen Turnbull. As with all sourcebooks it has a lot of information with a pretty decent section on the clans and the leaders of the era.
 
If you look up on the internet for the Ospreypublishing company you will find several books in their campaign series dealing with each individual battle of this period. And also in their Essential History series theres a good introductory book with some good maps showing the clans and their campaign movements of this period. The books are a bit costly at £13 or £15 but still worth it if youve interested. And there are also some books
by an author called Stephen Turnbull who is one of the leading specialist on this period dealing with the Samurai wars in Japan and their conquest of Korea as well. If you google these detail you might find things good luck!!
 
I'm curious to know the arguments from these historians about how Japan wasn't a feudal society. Most retainers held lands as fiefs by the will of their superiors, in exchange of service, income, and maintaining troops, and were held by oath, duty, or honor to serve his Lord to the death.

Here in the West, it is the core definition of feodality, from which the whole feudalism system originates. Its centered around the gesture of the hommage, which binds the overlord to his vassal, by the solemn promise to serve and being tied in service to a lord. The only difference I see in Japan, is that retainers didn't have to make a solemn oath about it, as service was the core definition of being a samurai in the first place.

I think the key issue to define the Feudalism in the west is that it was a two-way relationship between the lord and the vassal. It means that the lord gave or acknowledged the rights of the vassal to the lands that the vassal owned. It also meant that theoretically the vassals couldn´t be stripped off their lands at the whim of the liege lord while in Japan for example the Tokugawa shoguns moved their vassals from a fief to another fief.

In that way, the European Feudalism was based on a social contract. The Church, monarchs and their vassals were expected to follow that social contract and violating it would most likely have serious consequences. Japanese feudalism was based on loyalty to the liege lord and that gave the liege lord a lot power determining the fiefs of his vassals.

The vassals also wanted to have official protection against their liege lord whim´s in the Western World by defining the rights of the vassals in documents like Magna Charta. In Japan, there was no such development that I am aware of. In Japan vassals occasionally conspired against an incompetent liege lord while those vassals didn´t have any legal ways to limit the power of their incompetent liege lord.