Discussions Concerning Al-Mahdi ('a)
by Ayatullah Lutfullah as-Safi al-Gulpaygani
Written in the form of questions and answers, this book discusses the three main topics of Shi'ism, Imamah and Mahdism.
http://al-islam.org/discussions_mahdi
The Occultation of the Twelfth Imam - A Historical Background
by Jassim M. Hussain
A study of the occultation of the twelfth Imam ('a) using rarely consulted early Shi’ite sources, covering the time period from Imam Ja'far al-Sadiq ('a), the sixth Imam, to the end of the lesser occultation of the twelfth Imam ('a).
http://al-islam.org/occultation_12imam
The Awaited Savior of Humanity - al-Mahdi ('a) in the Eyes of the Ahlul Bayt
by Abdul Rahim Mugahi
Forty traditions on the twelfth Imam ('a), the awaited saviour.
http://al-islam.org/40ahadith-twelfthimam