The following books are recommended:
John Camp. The Athenian Agora: Excavations in the Heart of Classical Athens. Thames and Hudson, 1992.
John Camp. The Athenian Agora: The Short Guide to the Excavations. American School of Classical Studies, 2003.
Or download a digital version at:
http://www.agathe.gr/Texts/pdfs/PB16_English.pdf
These websites are worth a visit:
http://en.wikipedia.org/wiki/Ancient_Agora_of_Athens
http://www.agathe.gr/index.html
http://www.athensinfoguide.com/wtsagora.htm
For a virtual tour: