Sources of Japanese Tradition: From Earliest Times through the Sixteenth Century (that is, vol. 1) includes a chapter with the codes of several warrior houses from this period, as well as several chapters with texts from Nobunaga's time on. You might find it useful!

Edit: Also, about folkore, I'm sure plenty of people have done lots of research in the topic since then, but Yanagita Kunio is a classic. He recorded folk tales and local customs during the Meiji period in a manner very similar to the Grimm Brothers.

As far as I know there is no definitive book (in English) about the sengokujidai.

There are however several books dealing with aspects of it like Berry's Hideyoshi biography, Sadler's Tokugawa biography or Lamers' Nobunaga biography. A good general overview (though a bit dated) is presented in Sansom's A History of Japan, 1334-1615. Souyri's The world turned upside down presents an overview of society from the Heian period to the sengoku period.

Over in this question from a few days ago that I asked, one of the books recommended was The Last Samurai: The Life and Times of Taigo Takamori.

Another recommended source was from this other question that I didn't ask: Japan: A Documentary History, which I have yet to actually read, but is on my reading list.