A book I've found to be very beneficial is "Southern Forages" which was written by Dr. Donald Ball, Dr. Carl Hoveland and Dr. Garry Lacefield. It discusses warm and cool season forages, soil testing, fertilization, nutritional requirement, poisonous plants, matching forages to livestock enterprises, etc. It's not so advanced that the average livestock producer can't understand it. My book has a copyright of 1991 but is still very applicable today. I'm not sure where to tell you find one now but I heard Dr. Lacefield speak at a forage symposium about a year ago and he still had some there. May be able to find it on the web.