Websites

http://www.roman-emperors.org/aggieii.htm - discusses Agrippina's early life, and Gaius, Claudius and Nero's reign, in a large amount of detail

http://www.historyforkids.org/learn/romans/history/agrippina.htm - basic understanding, with simple language

http://www.jaysromanhistory.com/romeweb/ladycont/art4.htm - summary of Agrippina's life

Books

Agrippina the Younger, written by Ken Webb

Agrippina: Sex, Power, and Politics in the Early Empire, written by Anthony A. Barret