The two most popular sources for books on the Sengoku period seem to be Osprey books and the works of Stephen Turnbull, arguably one of the most knowledgeable western authorities on that period of Japan's history (and other periods as well).

Here is a link to the Osprey site. Try running a search on "samurai" and you will get a lengthy list of books, dominated by the works of Turnbull and Anthony J. Bryant.

http://www.ospreypublishing.com/

And here is a link to Anthony J. Bryant's website:

http://www.sengokudaimyo.com/

This should get you started. If you enjoy novels based in the Sengoku period, my first and foremost recommendation would be "Taiko" by Eiji Yoshikawa. It is the tale of Toyotomi Hideyoshi, from when he was a wee lad to the time he became Kampaku, or Taiko, of Japan (just prior to Tokugawa Ieyasu's victory and subsequent appointment as Shogun). Though it is fiction, it is heavily based in historical fact and features many of the characters present in the STW game. Yoshikawa is also Japanese--not a westerner--so you can get a nice feel for the Japanese perspective in the tale. The book reads beautifully and most who read it find it to be both entertaining and informative.

Hope this helps.