Good sources of info I have found on this is a book titled, "Breasetfeeding and Natural Child Spacing" by Sheila Kippley. As well as "The Art of Natural Family Planning" Both are available through www.ccli.org
NFP as explained in the above book consists of temperature and mucous observations, which after awhile tell you when you are ovulating so you may choose whether to aviod pregnancy or try to achieve pregnancy.
Nursing can affect your cycle depending on many factors - reference above books for more info on this. And you can ovulate, even without a full menstrual cycle. I have found the NFP method mentioned above to work very well for this because of the cross-referencing of my bodily signs.