It's difficult to find books in English about Naginata. I couldn't find any for quite a while, until I stumbled across an online copy of Illustrated Naginata. At times it's not very clear, but it's certainly a lot better than nothing!
I also have a hardcopy of Naginata: the definitive guide, which is much better. It took me a long time to find a copy because it is out of print. It is still available on the website above -- which I didn't use because they don't seem to ship affordably within Japan. I ended up finding a copy on amazon.co.jp but in the regular section (not in the English books section). It was mailed to me from Singapore.