Four Centuries of Influence of Iraqi Shiism on Pre-Safavid Iran 
An online book by Rasul Ja'fariyan explores the influence of the school of Sheikh Tusi of Baghdad on the survival of Shiism in Iran.
http://al-islam.org/mot/iraqishiism/Shi'a
A history of the Shia community, from its origins shortly after the death of the Prophet Muhammad through to the Islamic Republic of Iran.
http://www.wsu.edu:8080/~dee/SHIA/SHIA1.HTM
Shi'i Islam
Article on the Shia branch of Islam in Encyclopedia of the Orient; includes a summary of their history, with a genealogy of the Imams.
http://lexicorient.com/e.o/shii.htm
